Negative Thermal Expansion of Metal-Guanidinium Formates, Growth of up to cm-Sized Single Crystals, Heat Capacity, and Raman Spectroscopy

Abstract

We have synthesized and grown optical-quality single crystals of Cu²⁺, Zn²⁺, Mn²⁺, Co²⁺, and Mg²⁺ metal-guanidinium formate (MGuFo), [C(NH₂)₃]⁺[M²⁺(HCOO)₃]⁻ (where M²⁺ is the metal ion) up to cm-sized. In addition, we have measured their thermal expansion, Raman spectra, and heat capacities. The five compounds exhibit negative linear thermal expansion, and with the exception of CuGuFo, they preserve such behavior up to about 455 K. CoGuFo presents the lowest volume thermal expansion, the stiffest elastic coefficients, and the highest Debye temperature of all the studied compounds. Crystals with the ionic radius larger than ∼0.75 Å show a decrease in the Debye temperature and indicate a relative elastic softening as well as softening of Raman modes. Heat capacity measurements between 3.8 and 15 K indicate long-range ordering of spin-canted antiferromagnetism at low temperatures in CuGuFo, MnGuFo, and CoGuFo. Notably, we report for the first time the synthesis and crystal structure of MgGuFo, which is isostructural to MnGuFo, ZnGuFo, and CoGuFo. This study contributes to a deeper insight into structure–property relationships of metal-guanidinium formates by highlighting their pronounced anisotropic behavior, their negative thermal expansion, and the crucial influence of polyhedral rotations and framework distortions on their thermal and mechanical properties.

The synthesis, crystal growth, and physical properties of metal-guanidinium formates (MGuFo) with Cu²⁺, Zn²⁺ Mn²⁺, Co²⁺, and Mg²⁺ ions are reported. The study focuses on thermal expansion, heat capacity, and Raman spectroscopy, reporting the first synthesis of MgGuFo and analyzing how ionic radius influences thermal properties and structure−property relationships in these metal−organic frameworks.