Phase transitions in delafossite CuLaO2 at high pressuresNilesh P. Salke, Alka B. Garg, Rekha Rao, S. N. Achary, M. K. Gupta, R. Mittal, and A. K. Tyagi

Citation: Journal of Applied Physics 115, 133507 (2014); doi: 10.1063/1.4870577 View online: http://dx.doi.org/10.1063/1.4870577 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/115/13?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Structural phase transition of BaZrO3 under high pressure J. Appl. Phys. 115, 124907 (2014); 10.1063/1.4868906 Absence of phase transitions in an oxygen stoichiometric cobaltite, YBaCo4O7 AIP Advances 3, 022115 (2013); 10.1063/1.4792597 High-pressure lattice dynamical study of bulk and nanocrystalline In2O3 J. Appl. Phys. 112, 123511 (2012); 10.1063/1.4769747 Raman spectroscopy and field emission characterization of delafossite CuFeO 2 J. Appl. Phys. 107, 013522 (2010); 10.1063/1.3284160 High-pressure Raman scattering and x-ray diffraction of phase transitions in MoO 3 J. Appl. Phys. 105, 023513 (2009); 10.1063/1.3056049

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Phase transitions in delafossite CuLaO2 at high pressures

Nilesh P. Salke,1 Alka B. Garg,2 Rekha Rao,1,a) S. N. Achary,3 M. K. Gupta,1 R. Mittal,1

and A. K. Tyagi31Solid State Physics Division, Bhabha Atomic Research Center, Mumbai 400 085, India2High Pressure & Synchrotron Radiation Physics Division, Bhabha Atomic Research Center,Mumbai 400 085, India3Chemistry Division, Bhabha Atomic Research Center, Mumbai 400 085, India

(Received 6 February 2014; accepted 25 March 2014; published online 3 April 2014)

Structural stability of a transparent conducting oxide CuLaO2 at high pressures is investigated

using in-situ Raman spectroscopy, electrical resistance, and x-ray diffraction techniques. The

present Raman investigations indicate a sequence of structural phase transitions at 1.8 GPa and

7 GPa. The compound remains in the first high pressure phase when pressure is released.

Electrical resistance measurements carried out at high pressures confirm the second phase

transition. These observations are further supported by powder x-ray diffraction at high

pressures which also showed that a-axis is more compressible than c-axis in this compound.

Fitting the pressure dependence of unit cell volume to 3rd order Birch-Murnaghan equation of

state, zero pressure bulk modulus of CuLaO2 is determined to be 154(25) GPa. The vibrational

properties in the ambient delafossite phase of CuLaO2 are investigated using ab-initiocalculations of phonon frequencies to complement the Raman spectroscopic measurements.

Temperature dependence of the Raman modes of CuLaO2 is investigated to estimate the

anharmonicity of Raman modes. VC 2014 AIP Publishing LLC.

[http://dx.doi.org/10.1063/1.4870577]

INTRODUCTION

Transparent conducting oxides (TCO) of the delafossite

family AMO2 (A¼Cu, Ag; M¼Al, Sc, Ga, In) have many

unusual physical properties leading to technological applica-

tions in devices, such as solar cells, touch panels, as electro-

des in flat panel displays ultraviolet-emitting diodes.1 While

there are many n-type TCOs, p-type conductivity in transpar-

ent semiconductors is rare and essential for applications in

p-n junctions.2 Copper delafossites can be made p-type semi-

conductors either by doping or creating non-stoichiometry.

The origin of positive carriers in undoped delafossites is ei-

ther due to excess oxygen in the interstitials or copper vacan-

cies. Engineering of optical and electronic band structure by

doping in copper delafossites makes them useful as photoca-

talyst to produce hydrogen by water splitting.3 Catalytic ac-

tivity of copper delafossites also finds applications in

decomposition of toxic waste gases.4 Some members of the

delafossite family have attracted interest due to the multifer-

roic properties exhibited by them;5,6 wherein ferroelectricity

is induced by magnetic ordering.

Delafossite type of crystals have layered structure with

hexagonal P63/mmc or rhombohedral R�3m space group,

with a general formula AMO2, in which monovalent cations

A (A¼Cu, Ag) are linearly coordinated with two oxygen

ions along the c-axis and the trivalent cations M are octahe-

drally coordinated to oxygen atoms.7 Delafossite com-

pounds, where the trivalent ion is magnetic, have been

studied with the interest of magnetic transitions exhibited by

them. Both CuCrO2 and CuFeO2 show antiferromagnetism

below 24 and 14 K respectively.8,9 Though earlier magneto-

striction studies indicated a structural transition in CuCrO2,8

similar to the ferroelastic transition in CuFeO2,9 no anomaly

in either of them has been observed in low temperature

Raman spectroscopic studies.5 This suggests that the ferroe-

lastic transitions are not driven by Raman active modes.

Due to widely different coordination of the cations,

delafossites are expected to have a rich phase diagram,

especially many pressure induced phases. Earlier, high

pressure investigations of some of the members of this fam-

ily of compounds have revealed interesting phase transi-

tions. X-ray diffraction studies along with M€ossbauer and

x-ray absorption spectroscopy at high pressure have

revealed a sequence of reversible structural/electronic-mag-

netic transitions in CuFeO2.10 CuFeO2 transforms from

R�3m to C2/c at 18 GPa, while CuCrO2 transforms from

R�3m to P21/m at 26 GPa.10,11 Contrary to usual trend

observed for a class of materials under pressure, these com-

pounds do not seem to follow any specific trend in terms of

phase transition sequence; various copper delafossites

behave differently under pressure except in the case of

CuGaO2 and CuAlO2 where they seem to follow a trend.

From Raman spectroscopic and x-ray diffraction measure-

ments, it has been reported that both CuAlO2 and CuGaO2

transform to unresolved structures above 34 and 26 GPa,

respectively.12–15 In both CuAlO2 and CuGaO2, in-situExtended X-ray Absorption Fine Structure measurements

have confirmed that the transition involves change in cop-

per environment.14,15 Both CuAlO2 and CuGaO2 are indi-

rect band gap semiconductors. One common feature of all

the copper delafossites belonging to R�3m is that under high

a)Author to whom correspondence should be addressed. Electronic mail:

rekhar@barc.gov.in

0021-8979/2014/115(13)/133507/7/$30.00 VC 2014 AIP Publishing LLC115, 133507-1

JOURNAL OF APPLIED PHYSICS 115, 133507 (2014)

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pressure, a-axis was found to be more compressible than

the c-axis resulting in more regularization of oxygen octa-

hedra compared with the distorted octahedra at ambient

conditions.14–17 There have been several computational

studies on these compounds to understand their high pres-

sure behavior. Ab-initio calculations of phonon frequencies

under pressure have indicated that pressure induced phase

transitions in CuGaO2 and CuAlO2 are initiated by dynami-

cal instability of a transverse acoustic phonon.12,13

Electronic structure calculations have predicted change in

band gap with pressure in CuAlO2 with the compound

remaining an indirect band gap semiconductor up to

36 GPa, even though its smallest direct band gap decreases

with pressure.18 Recent first principles calculations of crys-

tal structures determined the critical pressure of transition

for delafossite CuAlO2 to a leaning delafossite with a

higher bandgap to be 60 GPa.19 Optical absorption studies

and ab-initio calculations of electronic structure on

CuScO2, a direct band gap semiconductor, reported changes

in electronic band structure, with direct gap abruptly

decreasing with increase in pressure at 18 GPa.20 Among

other copper delafossites, CuLaO2 is a direct gap semicon-

ductor with a band gap of 2.77 eV and p-type conductiv-

ity.21 Due to its lower band gap, CuLaO2 is expected be a

better candidate as a photoelectrode for hydrogen produc-

tion than other copper delafossites.22

Recently, negative thermal expansion (NTE) has been

reported in some members of the delafossite family.7

Interestingly, CuLaO2 exhibits NTE along both a- and

c-axes at low temperatures and it shows nearly isotropic vol-

ume thermal expansion, though its structure is anisotropic.

Neutron diffraction studies from 30 to 600 K range reported

that while a-axis shows positive expansion above 100 K,

c-axis contracts in the temperature range 30–200 K and

expands at higher temperatures up to 600 K.7 NTE in copper

delafossites in which Cu is two-fold coordinated, is attrib-

uted to the vibrational motion of Cu atom perpendicular to

the linear chain of O-Cu-O which is along the c-axis.23

High pressure investigation of NTE materials reveals a

variety of interesting phenomena, such as softening of pho-

nons, phase transitions, and also amorphization. High pres-

sure investigation of Raman spectrum gives information

about the mode Gr€uneissen parameter, useful for understand-

ing the thermal expansion behavior. Though Raman spec-

troscopy gives information about only the zone-center

phonons, it is quite useful in understanding the contribution

of different vibrations to the thermal expansion. Earlier pre-

liminary Raman spectroscopic studies on CuLaO2 at high

pressures reported pressure dependence of mode frequencies

in the ambient phase and a phase transition at 1.8 GPa.24

Here, we report detailed Raman spectroscopic studies, x-ray

diffraction, electrical transport, and ab-initio calculations of

phonon frequencies of CuLaO2 at high pressures.

EXPERIMENTAL DETAILS

The compound CuLaO2 was synthesized by solid state

reaction of Cu2O and La2O3. The sample was characterized

using powder x-ray diffraction technique. Unit cell

parameters and atomic coordinates obtained by Rietveld

refinement of x-ray powder diffraction data confirmed it to

be a delafossite structure with space group R�3m and lattice

parameters are in agreement with literature values.25 Raman

spectroscopic measurements at high pressure were carried

out from inside a diamond anvil cell (DAC) (Diacell B-05)

in back-scattering geometry, with 4:1 methanol-ethanol mix-

ture as pressure transmitting medium which remains hydro-

static up to 10 GPa.26 Spectrum of polycrystalline sample of

CuLaO2 was excited using 532 nm (2.33 eV) laser line of

power �15 mW. Pressure was measured using the ruby fluo-

rescence technique.27 Scattered light was analyzed using a

home built 0.9 m single monochromator,28 coupled with an

edge filter and detected by a cooled CCD. Entrance slit was

kept at 50 lm, which gives a spectral band pass of 3 cm�1.

Raman measurements in the temperature range 77–600 K

were carried out using the temperature stage from Linkam

(Model-THMS 600), also in back-scattering geometry.

The electrical resistance measurements under high pres-

sure were carried out on CuLaO2 in an opposed Bridgman

anvil setup. Bridgman assembly consists of 12 mm face diame-

ter tungsten carbide (WC) anvil pairs, two 0.2 mm thick pyro-

phylite (alumino silicate Al2O3.2SiO24H2O) gaskets with

central hole of diameter 3 mm and steatite as pressure transmit-

ting medium with in-situ bismuth pressure calibration. Initially,

the powdered sample was pressed between WC anvils to a load

of 3 ton. The well compacted material was then trimmed to

1.5 mm width and 2.5 mm length pieces with a thickness of

0.2 mm, for electrical resistance measurements. For four probe

resistance measurements, stainless steel wires of 40 lm diame-

ter were used. For each pressure point, 2 min pressure soaking

time was given before recording the resistance.

In-situ high pressure x-ray diffraction measurements

were carried out at the powder x-ray diffraction beam line of

Elettra synchrotron source, Italy. The data were collected in

angle dispersive x-ray diffraction (ADXRD) mode, in the

transmission geometry. The wavelength of the x-ray

employed and the sample to image plate (IP) distance were

calibrated using CeO2 diffraction pattern. Sample to detector

distance in the high pressure XRD set-up was 153.52 mm.

Hardened stainless steel gasket with a central hole of diame-

ter 150 lm and thickness 50 lm contained the sample. For

high pressure measurements, finely powdered CuLaO2 along

with gold as pressure calibrant and methanol–ethanol (4:1)

mixture as pressure transmitting medium were loaded in a

Mao–Bell-type DAC with diamond anvils of culets size

400 lm. X-ray powder patterns at various pressures were

collected employing x-ray of wavelength 0.5997 A colli-

mated to 80 lm diameter. Typical exposure times of 15–20

min were employed for measurements at high pressures.

Images of the powder diffraction rings were read from the

MAR345 image plate detector with a resolution of

100� 100 lm2 pixel size. The images thus obtained were

integrated using the program FIT2D.29

COMPUTATIONAL DETAILS

The first principles density functional theory methods

(DFT) for total energy and phonon calculations were carried

133507-2 Salke et al. J. Appl. Phys. 115, 133507 (2014)

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out using Vienna ab-initio simulation package (VASP)30,31

software. The generalized gradient approximation (GGA)

exchange correlation given by Perdew, Burke, and

Ernzerhof32,33 with projected–augmented wave method has

been used. The plane wave pseudo-potential with plane

wave kinetic energy cutoff of 780 eV is used in the calcula-

tion. The integrations over the Brillouin zone were sampled

on a 8� 8� 2 grid of k-points generated by Monkhorst-pack

method.34 The above parameters are found to be sufficient

for total energy convergence of less than 0.1 meV. Density

functional perturbation theory has been used for zone centre

phonon calculation implemented in VASP. The convergence

criteria for the total energy and ionic forces were set to

10�8 eV and 10�5 eV A�1, respectively.

RESULTS AND DISCUSSION

Raman spectroscopy

In the delafossite structure R�3m, primitive unit cell con-

sists of four atoms resulting in 12 normal modes which trans-

form as C¼A1gþEgþ 3A2uþ 3Eu of which Eg and A1g are

Raman active modes. Movement in the direction of Cu-O

bonds along the hexagonal c-axis is represented by A modes,

whereas E modes correspond to vibrations in the perpendicu-

lar direction. In the absence of single crystals, Raman modes

were identified by comparing the Raman spectra of analo-

gous compounds like CuAlO2 and CuGaO2,12,13 as well as

by using ab-initio calculations.

Figure 1 shows the evolution of Raman spectra of

CuLaO2 under high pressures. At ambient conditions (phase

PI), it consists of two modes at 318 and 652 cm�1 identified

as Eg and A1g, respectively. An asymmetry on the low fre-

quency side of the Eg mode and a few weak modes around

200 cm�1 are also observed at ambient conditions, which

could be non-zone center modes observed due to relaxation of

Raman selection rules due to copper vacancies or interstitial

oxygen, similar to those observed in CuAlO2/CuGaO2.12,13

The frequencies of both the Raman modes are found to

increase monotonically with pressure. Above 1.8 GPa, several

new modes appear in the low frequency region. On further

pressurization, the low frequency component of Eg mode

increased in intensity accompanied by softening of both the

components. Appearance of new modes and changes in the

pressure dependence of mode frequencies indicate a phase

transition to a phase PII. Above 1.8 GPa, the A1g mode could

not be followed. Appearance of many new modes in PII indi-

cates lower symmetry of the high pressure phase. Nature of

changes observed in the Raman spectra at 1.8 GPa are similar

to the observations in the Raman spectra of other delafossite

members CuAlO2 and CuGaO2, which show high pressure

transitions at 34 and 26 GPa, respectively.12,13 Higher phase

transition pressures in CuAlO2 and CuGaO2 could be attrib-

uted to the difference in ionic radii and polarizability of the

counter cations. Lanthanum has larger ionic radius and higher

polarizability compared with Al or Ga, the deformation of

coordination polyhedra and delocalization of electron density

around lanthanum can be expected at lower pressure. Thus,

the pressure induced transitions in these are related to the dif-

ferences in the bonding and electronic structure. This is fur-

ther supported from the behaviour of CuLaO2 at still higher

pressures. The Raman intensity of all the modes reduced dras-

tically above 7 GPa and the sample becomes opaque indicat-

ing drastic reduction in the band gap. The absence of any

detectable Raman spectra above 7 GPa could be due to phase

transition to a phase with no Raman active modes, amorphous

phase or the lack of Raman intensity could be due to change

in electronic band structure leading to increase in absorption.

Of these possibilities, the last possibility looks more feasible

since the sample becomes opaque. Finally, on complete

release of pressure from 8 GPa, sample remains in phase PII.

Figure 2 indicates the pressure dependence of Raman mode

frequencies. The pressure coefficient of all the modes in the

PI and PII phases is given in Table I. Frequencies of modes

that appear only in the phase PII are extrapolated to zero pres-

sure. Among the modes that appear in PII, the modes at

112 cm�1 and the two split components of Eg mode in the am-

bient phase around 318 cm�1 show softening with increase in

pressure whereas all the other modes show usual hardening.

FIG. 1. Raman spectra of CuLaO2 at various pressures. Red bars represent

the Raman mode frequencies as obtained from ab-initio calculations. Note

the appearance of new modes above 2 GPa.

FIG. 2. Pressure dependence of Raman mode frequencies of CuLaO2. The

solid lines are linear fit to data in a particular phase.

133507-3 Salke et al. J. Appl. Phys. 115, 133507 (2014)

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Ab-initio calculations of phonon frequencies were carried

out to assign the Raman modes. The vertical bars in Figure 1

show the calculated Raman frequencies in the phase PI, from

ab-initio calculations. Both calculated Raman frequencies

and the pressure dependence of Raman frequencies are given

in Table I. It may be noted that the frequencies of both the

modes as well as their pressure dependencies are found to be

under-estimated as compared with the experimental values.

The temperature dependence of Raman modes is investi-

gated to extract information about the anharmonicity of

Raman modes which plays an important role in the thermal

expansion of a material. Figures 3(a) and 3(b) show the

Raman spectra at various temperatures and Figure 4 shows

the temperature dependence of Raman modes in the range

77–593 K. There are no discontinuous changes in this

temperature range indicating absence of any phase transition.

Both the modes show normal anharmonic behavior with lin-

ear dependence indicating predominantly three phonon decay

process. Beyond 593 K, we did not observe any Raman

modes due to darkening of the sample, which may be prob-

ably due to the transformation of the sample to CuLaO2.62.35

The temperature dependence of phonon frequency arises

due to thermal expansion of the lattice and anharmonic inter-

actions between them. For an isotropic system, phonon fre-

quency can be considered as a function of volume and

temperature. The change in phonon frequency as a function

of temperature can be separated into a quasiharmonic contri-

bution which arises only due to change in volume, also

called “implicit anharmonicity” and a purely anharmonic

contribution (explicit anharmonicity) which arises due to

changes in vibrational amplitude.36 Raman spectroscopic

studies at high pressures and temperature are useful in sepa-

rating these two parts. At a particular pressure, the total

change in phonon frequency due to temperature can be

expressed as follows:

@xi

@T

� �p

¼ @xi

@V

� �T

@V

@T

� �p

þ @xi

@T

� �V

: (1)

This can also be written as

1

xi

@xi

@T

� �p

¼ �aciT þ1

xi

@xi

@T

� �V

(2)

where ciT ¼ � @lnxi

@lnV

� �T¼ B0

xi

� �@xi

@P

� �T

is the isothermal

Gr€uneissen parameter, a is the volume thermal expansion

coefficient, and B0 is bulk modulus.

The left-hand side of Eq. (2) gives the temperature de-

pendent isobaric frequency shift, which is the total anharmo-

nicity effect as measured in temperature dependent Raman

experiments. The first term on the right-hand side is the

TABLE I. Experimental and calculated Raman mode frequencies at room pressure and their pressure coefficients, experimental temperature coefficients and

the anharmonicity calculated using a Bulk modulus of B0¼ 154 GPa and thermal expansion coefficient a¼ 1.07�10�5 K�1. The numbers with “a” in the first

column indicate the new modes that appear in the high pressure phase PII which are obtained by extrapolation of high pressure data to ambient pressure.

PI

Mode

frequency, (cm�1)

@xi

@T

� �p

(cm�1 K�1)

@xi

@P

� �T

(cm�1 GPa�1)

Total

anharmonicity

(10�5 K�1)

Implicit

(pure-volume)

(10�5 K�1)

Explicit

(pure-temperature)

(10�5 K�1)

PII @xi

@P

� �T

(cm�1 GPa�1)

114a … … … … … �0.9(3)

116a … … … … … 1.6(1)

147a … … … … … 0.9(2)

191a … … … … … 2.4(3)

273 (Eg Cal.) … 4.1 … … … …

318(Eg) �0.0056(8) 5.0(7) �1.76 �2.59 0.83 �1.4(2)

324a … … … … … �3.0(8)

409a … … … … … 4.4(8)

620 (A1g Cal.) … 6.5 … … … …

652(A1g) �0.005(1) 9(1) �0.77 �2.27 1.50 …

695a … … … … … 1.2(9)

748a … … … … … �0.7(2)

FIG. 3. (a) and (b) Raman spectra of CuLaO2 at various temperatures.

133507-4 Salke et al. J. Appl. Phys. 115, 133507 (2014)

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implicit anharmonicity arises due to isothermal frequency

shift as a function of pressure. This is the volume contribu-

tion to the frequency shift. The second term on the right-

hand side of Eq. (2) is the explicit anharmonic contribution

to the frequency shift which is the pure-temperature effect.

The results as summarized in Table I, separate the total fre-

quency shifts into the pure-volume and pure-temperature

contributions of Raman active modes. We have used experi-

mental value of bulk modulus B0¼ 154 GPa obtained from

our high pressure x-ray diffraction experiments on CuLaO2,

which will be discussed later. Volume thermal expansion

coefficient a¼ 1.07� 10�5 K�1 in the range 300 K to 600 K

is used from Ref. 7. Though ideally, this formalism can be

applied only for isotropic systems, where the phonon fre-

quency is considered to be a function of volume and temper-

ature x (V, T), it has been applied for uniaxial systems,

where the phonon frequency depends also on c/a ratio.37 The

volume contribution so calculated has been found to differ

from the accurately calculated value as in Ref. 38 obtained

using formalism for uniaxial systems by about 10%. This is

because the relative change in c/a is an order of magnitude

less than the relative change in volume with pressure or tem-

perature.37 In the present case, in the absence of uniaxial

pressure data, the anharmonicity is estimated using the

expressions for cubic systems. As seen in the Table I,

pure-volume or the implicit contribution is dominant for

both the modes.

Resistance measurements

Figure 5 shows the pressure dependence of resistance for

CuLaO2. Since the measurements were done on compacted

polycrystalline sample along with electrical contacts via pres-

sure, initial value of resistance was quite high (> 20 MX).

Hence, we have plotted the data above 2 GPa. As pressure is

further increased, the resistance of the sample decreases at a

rate of 0.34 KX/GPa up to 5.3 GPa; beyond that the resistance

drops by three orders of magnitude indicating abrupt changes

in the material. This is around the pressure region of second

high pressure transition, where the intensity of Raman modes

is found to vanish. An abrupt change in resistance in a semi-

conductor under pressure mainly arises from a change in the

energy band gap, as the applied pressure changes the elec-

tronic band structure thereby changing the number of elec-

trons in the conduction band and holes in the valence band.39

Thus, the sharp decrease in the resistance beyond 5.3 GPa is

an intrinsic nature of the sample and can be attributed to the

abrupt decrease in band gap with increase in pressure. On fur-

ther increase of pressure up to 10 GPa (the highest pressure

reached in the present investigations), the resistance contin-

ued to decrease but with much slower rate. The pressure de-

pendence of electrical resistance correlates well with the

second high pressure phase transition involving decrease in

band gap at around the same pressure as disappearance of

Raman bands. The differences in transition pressures by two

techniques could be due to the presence of quasihydrostatic

conditions of resistance measurements where solid medium is

used as pressure transmitting medium.

High pressure X-ray diffraction

In order to confirm the phase transitions observed by

Raman spectroscopy, in situ x-ray diffraction experiments

were carried on CuLaO2 up to high pressures of 12 GPa.

Figure 6 shows the ADXRD pattern of CuLaO2 at various

pressures. Above 2.0 GPa, two new peaks appear in the pat-

tern in the low angle region indicative of a structural transition

corroborating the results obtained by Raman spectroscopy.

Beyond 7 GPa, there is a broadening of all the sample peaks

and a redistribution of intensity indicating onset of disorder.

FIG. 4. Variation of Raman mode frequencies with temperature.

FIG. 5. Variation of electrical resistance of CuLaO2 with pressure.

FIG. 6. Evolution of the diffraction pattern of CuLaO2 as a function of pres-

sure. * indicates the peaks due to the pressure marker. Arrow marks indicate

the new peaks that appear after transition to PII.

133507-5 Salke et al. J. Appl. Phys. 115, 133507 (2014)

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These observations are again consistent with the results of

Raman spectroscopic measurements. Rietveld analysis of the

x-ray diffraction data in the ambient phase reveals that as in

other delafossites, c/a ratio increases with pressure, as shown

in Figure 7(a). Figure 7(b) shows the variation of normalized

lattice parameter with pressure. The axial compressibility Ka

and Kc obtained using equation K ¼ � 1V

� �ð@V@PÞT are

2.5(1)� 10�3 GPa�1 and 1.04(7)� 10�3 GPa�1, respectively.

Similar to other delafossites, a-axis is found to be more com-

pressible than c-axis resulting in increase in c/a with pressure

which may make system unstable and lead to phase transition.

The unit cell volumes at different pressures were fitted with

the 3rd order Birch-Murnaghan equation of state (BM-EOS)

and are shown in Figure 7(c). Error in the volume is negligible

hence it cannot be seen in the figure, though it is plotted. The

zero pressure volume, bulk modulus, and pressure derivative

of bulk modulus obtained from the BM-EOS are 217.3(2) A3,

154(25) GPa, and 4.8, respectively. The error in estimated

bulk modulus is quite high due to coexistence of phases and

the small pressure range of stability of the ambient phase. As

in the case of other delafossites like CuAlO2 and

CuGaO2,14,15 we could not index the diffraction peaks corre-

sponding to the high pressure daughter phase and hence the

structures of high pressure phases could not be determined.

From the observed diffraction peaks of high pressure phase

PII, it can be mentioned that its structure is different from the

known high pressure phase of other delafossites like CuFeO2

and CuCrO2.10,11 As we do not have close data points in x-ray

diffraction measurements, the phase transition pressures are

estimated from Raman spectroscopy data. Phase transition

pressure as measured by x-ray diffraction measurements is in

general slightly higher, as compared to that Raman spectro-

scopic measurements, due to the fact that Raman spectroscopy

is a local probe whereas x-ray diffraction is a bulk tech-

nique.40 Note that changes in optical properties occur

at similar pressures as observed in the structural and vibra-

tional measurements, suggesting a correlation between all

phenomena.

In order to recover large volume of high pressure meta-

stable phase PII, we have also used the large volume

Bridgman anvil press to pressurize sufficient quantity of

sample with no pressure transmitting medium. The XRD

data were collected on as-synthesized sample and recovered

sample from non-hydrostatic compression of 10 GPa using

a standard diffractometer. Figure 8 shows XRD pattern of

CuLaO2 ambient phase and pressure quenched from

10 GPa, recorded with Mo Ka with k¼ 0.7107 A. Present

non-hydrostatic experiments show the same signatures of

the first high pressure phase as in hydrostatic, synchrotron

based XRD data confirming the absence of role of devia-

toric stresses in this phase transition. However, due to the

appearance of only two new clear peaks, coexistence of dif-

fraction peaks and broadening of all the peaks from ambient

structure, it was not possible to solve the high pressure

structure. The recovery of first high pressure phase after

pressure release has potential technological applications in

synthesis of new materials with tailored physical properties.

In order to understand the high pressure behavior of

delafossite compounds, the compressibility data of CuLaO2

obtained from present studies are compared with the reported

data of other delafossite compounds and they are presented

in Table II. It is noted that the physical parameters like bulk

modulus obtained experimentally for CuLaO2 agree well

with that of the other members of the delafossite family. The

anisotropic nature of compressibility in CuLaO2 is in tune

with that of other delafossites. The present results also give a

clue to systematic understanding of high pressure behavior

of copper delafossites. As proposed earlier24 in the family of

FIG. 7. (a) Variation of c/a with pressure in the ambient phase. (b) Variation

of normalized lattice parameters. (c) Variation of cell volume with pressure

in the ambient delafossite phase. Square symbols are experimental data

points and solid line is the fitted data to the third order Birch–Murnaghan

equation of state.

FIG. 8. XRD pattern of CuLaO2, as-synthesized (blue) and pressure

quenched from 10 GPa (red), recorded with Mo Ka with k¼ 0.7107 A.

Arrow marks indicate the diffraction peaks due to the first high pressure

phase.

133507-6 Salke et al. J. Appl. Phys. 115, 133507 (2014)

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delafossites CuMO2 (for M¼Al, Ga, and La), the variation

in the first transition pressure can be correlated to the ionic

radii of trivalent cation M3þ. The first transition pressure

decreases from 34 GPa in CuAlO2 to 1.8 GPa in CuLaO2 as

the radius increases from 0.54 to 1.03 A. We observe that the

first transition pressure is inversely proportional to the ionic

radius of M3þ in this class of delafossites CuMO2. Thus, a

general inference of the pressured induced phase transition is

related to the deformation of bonding and electronic struc-

ture around the of M3þ ion and hence the transition can be

attained at lower pressure for larger and highly polarizable

ions. Furthermore, the observation of phase transition

accompanied by a change in band gap in CuLaO2, at moder-

ate to lower pressure, will have merit to envisage applica-

tions like touch panel and sensors.

CONCLUSION

Raman spectroscopic studies point out towards two

phase transitions in CuLaO2 at 1.8 GPa and 7 GPa which are

further supported by electrical resistance and x-ray diffrac-

tion measurements. There is an increase in anisotropy with

pressure in the ambient phase, typical of delafossite family

of compounds. From x-ray diffraction measurements, bulk

modulus of CuLaO2 is determined to be 154(25) GPa. The

changes in the resistance at the second high pressure transi-

tion are indicative of a band gap collapse. The first transition

pressure is found to vary inversely as the ionic radius of M3þ

in copper delafossite CuMO2.

ACKNOWLEDGMENTS

The authors thank Dr. S. L. Chaplot and Dr. R.

Mukhopadhyay for support and encouragement. N.P.S.

acknowledges Department of Atomic Energy, India for

financial support.

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TABLE II. Compressibility data of delafossite family of compounds.

CuLaO2

CuAlO2

(Ref. 14)

CuGaO2

(Ref. 13)

CuFeO2

(Ref. 17)

CuCrO2

(Ref. 16)

B0 (GPa) 154(25) 200(10) 202(15) 156 126.8

Ka (10�3 GPa�1) 2.5(1) 2.06(5) 1.96(5) 2.58(4) 2.30(6)

Kc (10�3 GPa�1) 1.04(7) 0.83(4) 0.75(4) 0.65(2) 0.39(9)

133507-7 Salke et al. J. Appl. Phys. 115, 133507 (2014)

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