The complete elastic stiffness tensor of thiourea has been determined from thermal diffuse scattering (TDS) using high-energy photons (100 keV). Comparison with earlier data confirms a very good agreement of the tensor coefficients. In contrast with established methods to obtain elastic stiffness coefficients (e.g. Brillouin spectroscopy, inelastic X-ray or neutron scattering, ultrasound spectroscopy), their determination from TDS is faster, does not require large samples or intricate sample preparation, and is applicable to opaque crystals. Using high-energy photons extends the applicability of the TDS-based approach to organic compounds which would suffer from radiation damage at lower photon energies.

We present the synthesis, crystal structure, thermal expansion, Raman spectra, and heat capacity of Ca-guanidinium formate, a new member of the family of metal−organic perovskites ([C(NH 2 ) 3 ][M 2+ (HCOO) 3 ]), where M 2+ is a divalent metal ion. Ca-guanidinium formate shows an extraordinary strong negative thermal expansion between 100 and 400 K in the crystallographic (001) plane with thermal expansion coefficients α 11 = α 22 = −30.0(7) × 10 −6 K −1 . Perpendicular to this plane, along a 3 , the crystal structure of [C(NH 2 ) 3 ][Ca(HCOO) 3 ] expands very strongly with increasing temperature (α 33 = 155(2) × 10). This behavior is due to a reduction of the framework distortion on increasing temperature. From temperature-dependent Raman measurements we observed three soft Raman modes. The C−H out-of-plane bending mode (1062 cm −1 ) of formate groups is influenced by squeezing by guanidinium groups. This squeezing of formate groups has an effect of reducing the framework distortion and is asscociated with a strong negative thermal expansion. From heat capacity measurements we derived the Debye temperature θ Cp = 217 K. This study contributes to a deeper insight of structure− property relationships of compounds with strong anisotropic and large negative thermal expansion.

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We present for the first time the experimentally derived complete elastic stiffness tensors of orthorhombic metal guanidinium formates, ([C(NH 2 ) 3 ]-[M 2+ (HCOO) 3 ], MGFs), with M 2+ = Zn 2+ , Mn 2+ , and Cu 2+ which are structurally related to (ABX 3 )-perovskites using ultrasound techniques, Brillouin spectroscopy, and thermal diffuse scattering. Density functional theory calculations were also performed to calculate the elastic tensor of orthorhombic MGFs and extended to the study of trigonal MGFs with M 2+ = Ca 2+ and Cd 2+ . All MGFs studied here are very compressible, with bulk moduli ranging from 20 to 30 GPa. The MGFs possess a significant anisotropy in their elastic properties, which is extremely large, especially in the case of CdGF and CaGF.

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