Anisotropic thermal expansion in molecular solids: Theory and experiment on LiBH4

We propose a reliable and efficient computational method for predicting elastic and thermal expansion properties in crystals, particularly complex anisotropic molecular solids, and we apply it to the room-temperature orthorhombic Pnma phase of LiBH4. Using density-functional theory, we find thermal expansion coefficients at finite temperature, and we confirm them by temperature-dependent, in situ x-ray diffraction measurements. We also consider the effects of volume and pressure, as well as energy barriers for BH4- rotations and collective motions. Our combined study validates the theory and provides a better understanding of the structural behavior of LiBH4.