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The electronic structure and interatomic bonding of pyrophosphate crystal K2Mg (H2P2O7)2·2H2O are investigated for the first time showing complex interplay of different types
of bindings. The existing structure from single-crystal X-ray diffraction is not sufficiently
refined, resulting in unrealistic short O─H bonds which is rectified by high-precision
density functional theory (DFT) calculation. K2Mg (H2P2O7)2·2H2O has a direct gap of 5.22 eV and a small electron effective mass of 0.14 me. Detailed bond analysis between every pair of atoms reveals the complexity of various
covalent, ionic, hydrogen bonding and bridging bonding and their sensitive dependence
on structural differences. The K--O bonds are much weaker than Mg--O bonds and contributions
from the hydrogen bonds are non-negligible. Quantitative analysis of internal cohesion
in terms of total bond order density and partial bond order density divulges the relative
importance of different types of bonding. The calculated optical absorptions show
multiple peaks and a sharp Plasmon peak at 23 eV and a refractive index of 1.44. The
elastic and mechanical properties show features unique to this low-symmetry crystal.
Phonon calculation gives vibrational frequencies in agreement with reported Raman
spectrum. These results provide new insights indicating that acidic pyrophosphates
could have a variety of unrealized applications in advanced technology.
Published by the Royal Society under the terms of the Creative Commons Attribution
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