SCIENCE-41

Atomic Scale Study of Negative Thermal Expansion in Tungstate n EXECUTIVE SUMMARY

Perturbed angular correlation study in Zirconium tungstate shows the occurrence of more than one distinct Zr sites. Based on the values of quadrupole parameters it is interpreted that among the four sites observed, one site represents

regular ZrO6 and the other three sites are associated with contracted and distorted ZrO6 . Effective decrease in Zr-O bond

length is understood to be due to an increase in the fractions of ZrO6 octahedra which undergo contraction with , thus explaining negative thermal expansion of the system.

n OUTLINE

Zirconium tungstate ZrW28 O is a cubic compound (Cf. Fig.1) consisting of a three dimensional network of corner linked

ZrO6 octahedra and WO4 tetrahedra, with the latter having one non-bridging W-O bond. This system exhibits negative (A) thermal expansion (NTE) isotropically over the temperature range 4 to 1000 K, whose mechanism is yet to be completely understood. It was earlier understood that a large transverse vibration of the atom in the middle of the W-O-Zr linkage, which requires corresponding rotations of the polyhedra, is the primary origin of NTE in this material due to unconstrained feature of the structure. Common feature of the existing literature is a dominant role of strong Fig. 1 : Structure of zirconium tungstate. bonds for NTE leading to a volume reduction induced by inward rotation of polyhedra. For understanding NTE it is

essential to probe at Zr sites (as WO4 are more rigid than

ZrO6 ) using any local technique. We have carried out181 Hf/ Ta based Perturbed Angular Correlation (PAC) spectroscopy, a hyperfine technique, on

this system to probe local structure at ZrO6 . Using this technique Electric Field Gradient (EFG) tensors at the site(s) of occupancy of probe atoms are deduced. Principal component of EFG tensor is proportional to the quadrupole frequency (Q) and is inversely proportional to Zr-O bond length. Asymmetry parameter (ratio of relative change of other two components of EFG and the principal component) is sensitive to orientation distortion if any associated with

ZrO6 . PAC measurements were carried out using BaF2 based three detector, twin fast-slow coincidence spectrometer (Cf. Fig. 2) setup at MSD. Time resolution of the spectrometer is Fig. 2 : BaF2 based three detector TDPAC spectrometer that is around 660 ps, while the detectors are set for 133 and 482 setup at MSD. keV gamma rays. Closed Cycle Refrigerator is used for low temperature measurements while an inbuilt mini furnace is used for high temperature measurements up to 600 K. R(t) P(w) A few representative time dependent anisotropy spectra R(t) and their Fourier transform P(w) are shown in Fig. 3. 0.1 T =250 K Analysis of the spectra shows that there are four distinct Zr m sites experiencing distinct quadrupole parameters such as 0.0 0.05 quadrupole frequency, asymmetry parameters. The fraction which is predominant at 250 K is interpreted to be due to -0.1

probe atoms occupying regular ZrO6 . This is supported by a point charge model based computation of EFG carried out -0.2 0.00 by us. An increase in the fractions experiencing higher 0.1 0.03 quadrupole frequencies at higher temperature is seen T =560 K m in Fig. 3. 0.0 0.02 Shown in Fig.4 is the variation of the fractions with the

measurement temperature. It is seen that the fraction f1 -0.1 0.01

representing the regular ZrO6 octahedra decreases while the other fractions increase with temperature. PAC spectra and -0.2 0.00 the results in terms of variation of quadrupole parameters 02040 with temperature are repeatable at room temperature after 02 in-situ measurements till 600 K. This means that the observed quadrupole parameters are due to different time (ns) w (GHz)

equilibrium configurations that ZrO6 take as a consequence of inward rotations of polyhedra. Fig. 3 : Representative TDPAC spectra and Fourier transforms

82 SCIENCE-41 n NEGATIVE THERMAL EXPANSION IN ZIRCONIUM TUNGSTATE

Vast majority of materials have a positive coefficient of expansion and their volume increases upon heating. Unusual property of contraction of volume on heating (Negative thermal expansion) is seen in some . In these systems the contraction is small and anisotropic and occurs only over a small temperature range. But Zirconium tungstate exhibits isotropic negative thermal expansion over a large temperature interval 4 to 1000 K. This article is about understanding negative thermal expansion in Zirconium tungstate by studying the variation of quadrupole parameters associated at Zr sites of ZrO6 using Perturbed Angular Correlation (PAC) Spectroscopy. n FURTHER DESCRIPTION

Plotted at the top inset in Fig. 4 is the variation of dl/l with temperature. The mean value of Zr-O bond length is obtained as 0.04 1.0 0.02 the average of the negative cubic root of quadrupole frequencies PAC 0.00 weighted with respect to corresponding values of the fractions. -0.02

Slope of the plot represents the coefficient of thermal expansion 0.8 (dl/l) -0.04 (which is found to be negative having a value of approximately - -5 -1 280 350 420 490

6x10 K . 1 f 0.6 Temperature (K) This implies that the existence of higher frequency components and importantly an increase in the magnitudes of these fractions 0.4 is essential to obtain a negative slope in the variation of mean Zr-O bond length with temperature. This means that the existence of 0.5 fractions other than f1 which represent contracted and distorted 0.4

ZrO6 octahedra caused by the inward rotations of polyhedra is 0.3 2 essential in explaining negative thermal expansion in zirconium f 0.2 tungstate. 0.1 Thus this atomic scale study provides a proof that an effective contraction of distinct ZrO6 octahedra due to inward rotations of 0.2 3 polyhedra accompanied by distortions is a very important factor f contributing for the observed NTE in these systems. This study 0.4 establishes that distortions of polyhedra due to their inward 0.3 rotations play a very important role and it should be taken in to

4 0.2 account for understanding negative thermal expansion in f zirconium tungstate. 0.1 Detailed electronic structure based computations are required to 240 280 320 360 400 440 be done to find out the degree of distortions associated with ZrO6 octahedra based on the values of quadrupole parameters obtained Temperature (K) in the present study. Alternatively the present results could be taken as input parameters (i.e) by invoking the distortions Fig. 4 : Variation of fractions of probe atoms associated with of octahedra while modelling the results of X-ray/neutron distinct ZrO6 octahedra. Shown at the inset is the diffraction and EXAFS measurements reported in the literature to variation of the mean change in Zr-O distance with get the degree of distortions associated with polyhedra. respect to original length i.e dl/l with temperature n ACHIEVEMENT

This work shows that there exists four distinct Zr sites in ZrW28 O . Though the mean Zr-O length in each distinct ZrO6 increases there is an increase in the fraction associated with contracted and distorted ZrO6 octahedra. This explains the effective contraction of ZrO6 octahedra and hence the observed negative thermal expansion in this system. n PUBLICATIONS ARISING OUT OF THIS STUDY AND RELATED WORK

R. Govindaraj, C. S. Sundar and A. K. Arora,Phys. Rev. B .,76 (2007) 12104.

Further inquiries: Dr. R. Govindaraj, Materials Science Division Metallurgy and Materials Group, IGCAR, e-mail: [email protected]

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