Structure of Ca(BD₄)₂ β-Phase from Combined Neutron and Synchrotron X-ray Powder Diffraction Data and Density Functional Calculations

Abstract

We have investigated the crystal structure of Ca(BD4)2 by combined synchrotron radiation X-ray powder diffraction, neutron powder diffraction, and ab initio calculations. Ca(BD4)2 shows a variety of structures depending on the synthesis and temperature of the samples. An unknown tetragonal crystal of Ca(BD4)2, the beta phase has been solved from diffraction data measured at 480 K on a sample synthesized by solid-gas mechanochemical reaction by using MgB2 as starting material. Above 400 K, this sample has the particularity to be almost completely into the beta phase of Ca(BD4)2. Seven tetragonal structure candidates gave similar fit of the experimental data. However, combined experimental and ab initio calculations have shown that the best description of the structure is with the space group P4(2)/m based on appropriate size/geometry of the (BD4)tetrahedra, the lowest calculated formation energy, and real positive vibrational energy, indicating a stable structure. At room temperature, this sample consists mainly of the previously reported alpha phase with space group Fddd. In the diffraction data, we have identified weak peaks of a hitherto unsolved structure of an orthorombic gamma phase of Ca(BD4)2. To properly fit the diffraction data used to solve and refine the structure of the beta phase, a preliminary structural model of the gamma phase was used. A second set of diffraction data on a sample synthesized by wet chemical method, where the gamma phase is present in significant amount, allowed us to index this phase and determine the preliminary model with space group Pbca. Ab initio calculations provide formation energies of the alpha phase and beta phase of the same order of magnitude (delta H < or = 0.15 eV). This indicates the possibility of coexistence of these phases at the same thermodynamical conditions.

Pressure-Temperature Phase Relations in Complex Hydrides

“…At room temperature, an orthorhombic Fddd α phase dominates [69], while heating to above 400 K produces a tetragonal P4 2 /m β phase [70]. A third γ phase, tentatively indexed as orthorhombic Pbca, has also been reported [70], but nothing is yet known about the high-pressure evolution of these structures. Ca(AlH 4 ) 2 , containing 7.8 percent hydrogen, has also been discussed as a possible hydrogen source.…”

Ionic conductivity and the formation of cubic CaH2 in the LiBH4–Ca(BH4)2 composite

“…Į-Ca(BH 4 ) 2 has an orthorhombic structure (Fddd [35] or F2dd [43]). ȕ-Ca(BH 4 ) 2 has a tetragonal crystal structure (P-4 [43] or P4 2 /m [44]). ȕ-Ca(BH 4 ) 2 is sometimes referred to as a high-temperature polymorph, as it has been observed to become the dominant phase above 130°C [44].…”

“…ȕ-Ca(BH 4 ) 2 has a tetragonal crystal structure (P-4 [43] or P4 2 /m [44]). ȕ-Ca(BH 4 ) 2 is sometimes referred to as a high-temperature polymorph, as it has been observed to become the dominant phase above 130°C [44]. Ȗ-Ca(BH 4 ) 2 has an orthorhombic crystal structure (Pbca) [45].…”

Synthesis and Characterization of NaBD3H, A Potential Structural Probe for Hydrogen Storage Materials

“…Moreover, promising compounds such as Ca(BH 4 ) 2 undergo several phase transitions with increasing temperature, and the structural characterization of all phases remains to be completed. [2][3][4] In particular, the accurate determination of boron hydrogen bond length in solids is experimentally difficult.…”