  SbPO4 is a complex monoclinic layered material characterized by a strong
activity of the non-bonding lone electron pair (LEP) of Sb. The strong cation
LEP leads to the formation of layers piled up along the a-axis and linked by
weak Sb-O electrostatic interactions. In fact, Sb is 4-fold coordination with O
similar to what occurs with the P-O coordination, despite the large difference
of ionic radii and electronegativity between both elements. Here we report a
joint experimental and theoretical study of the structural and vibrational
properties of SbPO4 at high pressure. We show that SbPO4 is not only one of the
most compressible phosphates but also one of the most compressible compounds of
the ABO4 family. Moreover, it has a considerable anisotropic compression
behavior with the largest compression occurring along a direction close to
a-axis and governed by the compression of the LEP and the weak inter-layer Sb-O
bonds. The strong compression along the a-axis leads to a subtle modification
of the monoclinic crystal structure above 3 GPa leading from a 2D to a 3D
material. Moreover, the onset of a reversible pressure-induced phase transition
is observed above 9 GPa, which is completed above 20 GPa. We propose that the
high-pressure phase is a triclinic distortion of the original monoclinic phase.
The understanding of the compression mechanism of SbPO4 can aid in
understanding the importance of the ion intercalation and catalytic properties
of this layered compound.
