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Temperature and High-pressure dependency of Infrared modes of Zn(CN)2, a negative thermal
expansion material
SOLEIL Company Contents > All the news > News 2009 > NTE

Negative thermal expansion (NTE) materials, such as ZrW2O8 and other related compounds are of intense current interest and many experimental and theoretical investigations have been reported and they emphasize the role played by the low energy phonons to explain this unusual behavior [1,2]. These low energy modes, known as Rigid Unit Modes (RUM) represent the transverse vibrational displacements of the O atom in the Zr-O-W linkage between the octahedral ZrO6 and tetrahedral WO4 rigid units. Recently a new non-oxide material Zn(CN)2 has been reported to have isotropic NTE over the temp range 25-375K [3] which is twice that of the best known material ZrW2O8. This material consists of three dimensional interpenetrating tetrahedral network of Zn-CN-Zn′ linkages(Fig. 1).

 


Fig. 1: Structure of Zn(CN)2

High-pressure IR spectroscopic studies of these materials are of special interest because they enable determination of the Gruneisen parameter which is directly related to the thermal expansion coefficient.
The experiment, carried out by Dr. Sudip DEB, from Raja Ramanna Centre for Adv. Tech., Indore ( India), Dr. T.N. Sairam from, Indira Gandhi Centre for Atomic Research (India) and Dr Alain Polian from Inst. de Minéralogie et Physique des Milieux Condensés, Paris, (Fig.2) uses two different high pressure cells ( one with diamond windows- DAC, and one with moissanite-MAC), with pressure varying up to 10 GPa, and one cooling stage (Linkam) from 77K to room temperature (Fig.3). Thanks to the set up at the SMIS beamline, Mid-IR and Far-IR microspectroscopy can be carried out on the same sample.


Fig.2 : From left to right: Dr. S. DEB, Dr. T.N. Sairam
and Dr A. Polian at SMIS beamline


Fig.3 : Linkam heating/cooling device used in this study.
ZnSe windows have been used for mid-IR measurement,
while HDPE( High Density PolyEthylene) was used for far
infrared measurement

The temperature dependency of mid-IR and far-IR phonon modes of Zn(CN)2, at ambient pressure is exemplified in Fig.3, while in Fig.4 is reported similar information from the pressure dependency at room temperature.


The C-N stretch mode at about 2217 cm-1displays an upward frequency shift on decreasing the temperature, while the same behavior is observed for one low frequency. phonon, and no shift is observed for the other low frequency.mode. A new band appears at low frequency which could possibly be interpreted as a phase transition.


Increasing the pressure induces an upward shift of the C-N stretch and the phonon mode at ~450 cm-1. Interestingly, the low frequency mode, which was not affected by temperature, is markedly affected by pressure increase, as it shifts to lower frequency

Thanks to the wide frequency range available at the microscopy beamline SMIS, a thorough investigation of the temperature and pressure dependency could bring very valuable information about the negative thermal expansion of this compound.

Authors would like to thank Jean Paul Itie, from SOLEIL, for his assistance during the measurements.


References :
1. T.A. Mary, J.S.O. Evans, T. Vogt and A.W. Sleight, Science, 272, 90 (1996)
2. A. W. Sleight, Annu. Rev. Mater. Sci. 28, 29-43 (1998)
3. A.L. Goodwin, C.J. Kepert, Phys. Rev. B 71 (2005) 14030

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