Bonding and Suppression of a Magnetic Phase Transition in EuMn 2 P 2 .

Electrons in solids often adopt complex patterns of chemical bonding driven by the competition between energy gains from covalency and delocalization, and energy costs of double occupation to satisfy Pauli exclusion, with multiple intermediate states in the transition between highly localized, and magnetic, and delocalized, and nonmagnetic limits. Herein, we report a chemical pressure-driven transition from a proper Mn magnetic ordering phase transition to a Mn magnetic phase crossover in EuMn 2 P 2 the limiting end member of the EuMn 2 X 2 (X = Sb, As, P) family of layered materials. This loss of a magnetic ordering occurs despite EuMn 2 P 2 remaining an insulator at all temperatures, and with a phase transition to long-range Eu antiferromagnetic order at T N ≈ 17 K. The absence of a Mn magnetic phase transition contrasts with the formation of long-range Mn order at T ≈ 130 K in isoelectronic EuMn 2 Sb 2 and EuMn 2 As 2 . Temperature-dependent specific heat and 31 P NMR measurements provide evidence for the development of short-range Mn magnetic correlations from T ≈ 250-100 K, interpreted as a precursor to covalent bond formation. Density functional theory calculations demonstrate an unusual sensitivity of the band structure to the details of the imposed Mn and Eu magnetic order, with an antiferromagnetic Mn arrangement required to recapitulate an insulating state. Our results imply a picture in which long-range Mn magnetic order is suppressed by chemical pressure, but that antiferromagnetic correlations persist, narrowing bands and producing an insulating state.