The Long‐Periodic Loop‐Branched Chain Structure of the Oxonitridophosphate La(21)P(40)O(46)N(57), Elucidated by a Combination of TEM and Microfocused Synchrotron Radiation

The lanthanum oxonitridophosphate La(21)P(40)O(46)N(57) was synthesized by high‐pressure metathesis from partially hydrolysed LiPN(2) and LaCl(3) at 750–950 °C and 7–9 GPa. The combination of transmission electron microscopy (TEM) and diffraction using microfocused synchrotron radiation revealed a monoclinic crystal structure (space group P2(1)/n, a=14.042(4), b=7.084(3), c=41.404(10) Å, β=97.73(3)° and Z=2), which is characterized by loop‐branched 21 member single chains of P(O,N)(4) tetrahedra that extend along [2 0 1]. These chains are related to the loop‐branched dreier single chains with dreier‐ring loops in stillwellite (CeBSiO(5)). In La(21)P(40)O(46)N(57), these chains are characterized by a complex long‐periodic conformation and exhibit disorder that involves La/N and P split positions. This is an extraordinarily long periodicity with respect to branched single chains of tetrahedra. La(21)P(40)O(46)N(57) constitutes the first rare‐earth oxonitridophosphate exhibiting a chain structure. Single‐crystal data are consistent with electron and powder X‐ray diffraction.