Origin of negative thermal expansion in Zn₂GeO₄ revealed by high pressure study

Abstract

Zn 2 GeO 4 , as an open-framework structure compound, exhibits negative thermal expansion (NTE) below room temperature. In this work, we investigated the structural stability and phonon modes employing the x-ray diffraction and Raman spectroscopy under high pressure up to 23.0 GPa within a diamond anvil cell, and we observed that a pressure-induced irreversible amorphization took place around 10.1 GPa. Bulk modulus, pressure coefficients, and Grüneisen parameters were measured for the initial rhombohedral structure. Several low-frequency rigid-unit modes are found to have negative Grüneisen parameter, which accounts for the primary part of NTE in Zn 2 GeO 4 . These results further confirm the hypothesis that the pressure-induced amorphization and the negative thermal expansion are correlated phenomena.

A thorough investigation of the crystal structure of willemite‐type Zn2GeO4

“…This is mostly due to the well-known overbinding at the PBE level of theory, improves significantly when switching to the more accurate hybrid functional HSE06, and has been observed for other oxide semiconductors before. [37,38] The bulk modulus B 0 and its pressure derivative B' 0 are 106.4 GPa (117.3 GPa) and 2.6 (2.1) at the PBE (HSE06) level of theory, and are in fair agreement to experimental results of 117.8 GPa and 4 [27] and 151.7 GPa and 4, [12] respectively.…”

“…[6][7][8] The susceptibility for photo-activation also qualifies the material as potential photocatalyst system, [9,10] and last but not least, the fact that the material has a very small thermal expansion that is, in fact, negative at lower temperatures has also attracted some interest. [11,12] Further to its inherent properties and potential uses, Zn 2 GeO 4 is being used as precursor material for the synthesis of ternary nitride [13] and oxide nitride materials [14][15][16] that are currently under investigation for photovoltaic [17,18] and photoelectrochemical applications. [19] The crystal structure of Zn 2 GeO 4 at ambient conditions is isostructural to Zn 2 SiO 4 (willemite, space group R � 3, no.…”

“…The inorganic crystal structure database contains only two crystal structure refinements for the pure Zn 2 GeO 4 , namely collection codes 16173 [20] and 68382, [21] and a more recent one on Tb 3 + doped Zn 2 GeO 4 , namely collection code 259671, [26] respectively. There are however, some other crystal structure refinements available, e. g., from Yang et al [27] and Cheng et al [12] In order to advance our current knowledge, we applied a combination of X-ray and neutron diffraction techniques in order to re-determine the crystal structure of Zn 2 GeO 4 , and supplemented the experimental results with first-principles calculations based on density functional theory (DFT). The combined efforts brought about a more comprehensive understanding of the structural features present in Zn 2 GeO 4 and provided an insight on the effects behind the observed features of the material.…”

Negative Thermal Expansion of Carbon Nanotube Bundles

“…A negative coefficient of thermal expansion can appear only in materials with a special structure that compensates for the asymmetry of interatomic interactions. In the literature, various mechanisms for the appearance of negative thermal expansion have been described, [1][2][3][4][5][6][7][8] including transverse vibrations of 2D materials [9] or the presence of low-frequency vibrational modes of structural units with rotational degrees of freedom, [10] phase transitions, [11] and chemical modifications. [12,13] Metal nanowires can exhibit negative thermal expansion (NTE) due to elastic softening, as well as due to the effect of surface stress.…”

Linear Zero Thermal Expansion in a Deep-Ultraviolet Transparent Crystal of BPO4 with Cristobalite-like Structure

“…The VTRS were collected to further confirm the mechanism of the linear ZTE behavior of BPO 4 . It is because the thermal expansion property is tightly correlated with the anharmonicity of lattice vibrations, which can be characterized from the variation of the phonon frequency with respect to temperature. ,− As shown in Figure b, for all measured temperatures there are seven principal peaks observed in the Raman spectrum (Modes I–VII in Figure b and Table S4). Six of them (Modes I and III–VII) are significantly softened as temperature increases, while the frequency of the mode around 459 cm –1 (Mode II) exhibits very small shifting with respect to temperature, indicating its key role in the linear ZTE behavior in BPO 4 .…”