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Scandium trifluoride maintains a cubic ReO(3) type structure down to at least 10 K, although the pressure at which its cubic to rhombohedral phase transition occurs drops from >0.5 GPa at ∼300 K to 0.1-0.2 GPa at 50 K. At low temperatures it shows strong negative thermal expansion (NTE) (60-110 K, α(l) ≈ -14 ppm K(-1)). On heating, its coefficient of thermal expansion (CTE) smoothly increases, leading to a room temperature CTE that is similar to that of ZrW(2)O(8) and positive thermal expansion above ∼1100 K. While the cubic ReO(3) structure type is often used as a simple illustration of how negative thermal expansion can arise from the thermally induced rocking of rigid structural units, ScF(3) is the first material with this structure to provide a clear experimental illustration of this mechanism for NTE.

A new phase, cubic ZrMo2O8, has been prepared by the low-temperature dehydration of ZrMo2O7(OH)2·2H2O. This material displays isotropic negative linear thermal expansion (α = −5.0 × 10-6 K-1) over a large temperature range. Unlike the previously reported cubic ZrW2O8, it does not undergo any phase transformations on heating at atmospheric pressure, and it does not display any pressure-induced phase transformations below 0.6 GPa at room temperature.

CaZrF6 and CaHfF6 display much stronger negative thermal expansion (NTE) (α L100 K ∼ −18 and −22 ppm·K–1, respectively) than ZrW2O8 and other corner-shared framework structures. Their NTE is comparable to that reported for framework solids containing multiatom bridges, such as metal cyanides and metal–organic frameworks. However, they are formable as ceramics, transparent over a wide wavelength range and can be handled in air; these characteristics can be beneficial for applications. The NTE of CaZrF6 is strongly temperature-dependent, and first-principles calculations show that it is largely driven by vibrational modes below ∼150 cm–1. CaZrF6 is elastically soft with a bulk modulus (K 300K) of 37 GPa and, upon compression, starts to disorder at ∼400 MPa. The strong NTE of CaZrF6, which remains cubic to <10 K, contrasts with cubic CoZrF6, which only displays modest NTE above its rhombohedral to cubic phase transition at ∼270 K. CaZrF6 and CaHfF6 belong to a large and compositionally diverse family of materials, AIIBIVF6, providing for a detailed exploration of the chemical and structural factors controlling NTE and many opportunities for the design of controlled thermal expansion materials.

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Variable temperature single-crystal structure analyses for Cs8Na16Si136, Rb8Na16Si136, Cs8Na16Ge136, and Rb8Na16Ge136 are reported along with electrical and thermal transport measurements on two polycrystalline specimens. The strong temperature dependence of the atomic displacement parameters for the alkali-metal atoms is indicative of significant disorder associated with the “rattling” alkali-metal atoms inside the two different polyhedra (sixteen dodecahedra and eight hexakaidecahedra per cubic unit cell) that makeup the type II clathrate hydrate framework. This disorder can lead to low lattice thermal conductivities. Transport measurements show these compounds to be metallic. The potential of type II clathrates for thermoelectric applications is discussed.

The cubic ReO 3 -type material ScF 3 exhibits strong isotropic negative thermal expansion (NTE) over a wide temperature range while remaining cubic. Control of its thermal expansion was investigated by forming Sc 1-x Ti x F 3 solid solutions, which were characterized by synchrotron powder diffraction at ambient pressure from 100 to 500 K. TiF 3 is fully soluble in ScF 3 at a synthesis temperature of 1338 K. The temperature for the cubic-torhombohedral phase transition in Sc 1-x Ti x F 3 varies linearly with composition (above 100 K), and, at large x, the transition is clearly first-order. The rhombohedral phase for each composition examined exhibits strongly positive thermal expansion, while the expansion of the cubic phase (between 420 and 500 K) is negative for all Sc 1-x Ti x F 3 .

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