Structural and Vibrational Properties of Calcium, Strontium, Barium, and Lead Borophosphates from First Principles
摘要
It is shown by first-principles calculations within the density functional theory (B3LYP hybrid functional and a basis set of localized orbitals implemented in the CRYSTAL package) that the CaBPO5 and SrBPO5 borophosphates can adopt both P3121 and P3221 symmetries, whereas PbBPO5 and BaBPO5 tend to adopt the former and the latter symmetry, respectively. Three non-equivalent oxygen atoms participate in the formation of BO4 (O1, O3) tetrahedral borates, forming chains via O3 atoms, and PO4 (O1, O2) phosphates, forming chained BPO5 complexes via common O1 atoms. The latter complexes contain A cations (A = Ca, Sr, Ba, Pb), each surrounded by ten oxygen atoms AO10, two O2 pairs (nearest), O1 atom, and one pair of O3 atoms. This structure is manifested in the calculated IR, reflection, and Raman spectra whose individual bands correspond to the vibrations of structural groups with a main contribution from individual oxygen atoms. The chain structure ensures stable behavior under hydrostatic pressure with a bulk modulus above 90 GPa. Nonuniform compression of intramolecular bonds and A–O distances causes different modes of Grüneisen parameters characterizing the behavior of vibrational spectra under pressure.