Possible Peierls Distortion Through Re₂ Dimer Formation in the LaNiO₂-Type Nitrides LnReN₂ (Ln = Pr, Nd)

Nitride perovskites are a recently opened class of materials with a limited number of stable compounds. Notably, anion-vacancy-ordered materials such as LaNiO₂-type nitrides are rare despite useful properties such as superconductivity reported for analogous oxides. Here, we report the preparation of two LnReN₂ (Ln = Pr, Nd) materials with a distorted LaNiO₂-type structure at high-pressure, high-temperature conditions of a large volume press. Powder X-ray and neutron diffraction show that the orthorhombic distortion (o-LnReN₂) of the LaNiO₂-aristotype structure is caused by buckling of the ReN_4/2 layers through Re dimerization. Electronic structure analysis reveals that the o-LnReN₂ materials are composed of classic nitridometallate anions and a cationic intermetallic framework. Moreover, the calculations reveal the driving force of the Re dimerization likely to be from an electronic instability related to a Peierls-type distortion. We further characterize the magnetic ground state of both materials with magnetization measurements as well as magnetic neutron scattering uncovering a long-range antiferromagnetic order of Nd³⁺ moments in NdReN₂ at T_N = 15.5 K. The stability of the compounds is investigated by temperature-dependent powder X-ray diffraction showing decomposition of NdReN₂ at ca. 750 °C. The o-LnReN₂ materials represent a curious class of nitrides at the border of classic nitridometallates, intermetallics, and low-dimensional complex chemistry.