Abstract
Structures of hypoxanthinium nitrate monohydrate crystals were determined at 15 different temperatures ranging from 20 to 285 K. Crystals undergo a phase transition from the twinned, low temperature phase of P21/n symmetry into the high temperature Pmnb phase. Sets of X-ray diffraction data acquired at each temperature were subjected to different types of refinement. We found that, as the temperature increases, the layers of hypoxanthinium nitrate monohydrate shift against each other, leading to the negative thermal expansion in one direction and positive one perpendicular to it. Such a phenomenon stops at the point of phase transition when the asymmetry in interactions in the structure disappears.
Highlights
When the temperature increases, we usually suspect the volume of the material to increase, which is a reasonable conclusion with a plethora of experimental evidence to support it physicochemical tables with thermal expansion and contraction coefficients of various materials consist mostly of positive values.[1,2] The explanation for this lays in the anharmonicity of the vibrations of atoms and ions inside the material
The high temperature (HT) structure was reported for the first time by Schmalle et al.[18]. Both HT and low temperature (LT) structures were already discussed in our previous work,[19] but there we focused on structures themselves and did not delve into the matter of phase transition nor thermal expansion
We decided to discuss only results obtained with Transferable Aspherical Atom Model (TAAM) and periodic DFT method (pDFT) optimization
Summary
We usually suspect the volume of the material to increase, which is a reasonable conclusion with a plethora of experimental evidence to support it physicochemical tables with thermal expansion and contraction coefficients of various materials consist mostly of positive values.[1,2] The explanation for this lays in the anharmonicity of the vibrations of atoms and ions inside the material. The HxA···NO3A and HxA···NO3D2 dimers have comparable energies calculated for each geometry (both Ees and Etot), but in every plot, the HxA···NO3B1 dimer has visibly less negative energy this is the same dimer with the largest change of hydrogen bond length that was discussed previously Such an outcome is apparently not influenced by the difference of distances between the molecules forming the dimer (they differ by only 0.5 Å, and the HxA···NO3B1 is not the one with the largest distance; see Figure 8) but rather by the “asymmetricity” of the bifurcated hydrogen bonds. There is very little change to energies from different temperatures for pDFT geometry, and if there is any, it is in accordance to slight changes in distances between the centers of mass of molecules forming the dimer. We believe that the way we approached the analysis of hypoxanthinium nitrate monohydrate can be a first step to similar investigations concerning other crystal structures, that were not previously screened for possible NTE
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