Abstract
A new pyroxene NaMnGe2O6 has been synthesized at 3 GPa and 800 °C, and fully characterized by x-ray single-crystal diffraction and neutron powder diffraction, measurements of magnetization and specific heat. Like other majority sodium pyroxenes, NaMnGe2O6 crystallizes into a monoclinic C2/c structure with unit-cell parameters a = 9.859(2) Å, b = 8.7507(18) Å, c = 5.5724(11) Å, and β =105.64(3)° at room temperature. The crystal structure is featured by quasi-one-dimensional chains of skew edge-sharing MnO6 octahedra running along the crystallographic c axis; these chains are connected by non-magnetic GeO4 tetrahedra, so as to lead to a low-dimensional magnetism. The highly distorted MnO6 octahedron consisting of three Mn-O bond lengths, i.e. 1.918 Å, 1.991 Å, and 2.198 Å, is consistent with the Jahn-Teller effect at Mn3+ in a cubic crystal field. A long-range cooperative Jahn-Teller distortion is formed by ordering longest Mn-O bonds between two neighboring octahedra along the chain direction. No orbital order-disorder transition has been found up to 750 K as checked by magnetic susceptibility. Like other alkali-metal pyroxenes with S > ½, NaMnGe2O6 (S = 2) was found to undergo a long-range antiferromagnetic ordering at TN = 7 K at low magnetic field due to the exchange interactions along and between chains. Due to the peculiar structural features and the corresponding magnetic coupling, the weak AF spin ordering gives way to a ferromagnetic-like state at a sufficiently high magnetic field. Specific-heat measurements demonstrated that a large portion of the magnetic entropy, i.e. > 60 %, has been removed above TN as a result of strong spin correlations within the quasi-one-dimensional Mn3+-spin chains. Neutron powder diffraction study suggests a commensurate magnetic structure defined by k = [0 0 0.5] with Mn moments aligned along the c axis. The present study on NaMnGe2O6 completed the evolution of magnetic properties as a function of the d-orbital occupancy from d1 to d5 in the magnetic pyroxenes.
