Charge and orbital order in frustrated Pb₃Mn₇O₁₅

Abstract

The candidate magnetoelectric Pb(3)Mn(7)O(15) has a structure consisting of one-third filled kagome layers linked by ribbons of edge sharing octahedra in the stacking direction. Previous reports have indicated a complex hexagonal-orthorhombic structural transition upon cooling through ~335 K, although its origins are uncertain. Here both structures are revisited using a combination of neutron and synchrotron x-ray diffraction data. Large shifts of oxygen positions are detected, which show that the interlayer sites and those which occupy voids in the kagome lattice are trivially charge ordered in both phases. The symmetry breaking is found to occur due to Mn(3+) orbital ordering on the ribbon sites and charge ordering of the subset of layer sites which make up a kagome network.

Spin and dipole order in geometrically frustrated mixed-valence manganite Pb3Mn7O15

“…As for the singlet partial spectrum (A = 9(1) %) it is likely due to an impurity phase containing Fe 3? ions in the four-fold oxygen coordination with cubic symmetry. These data are consistent with earlier reported Mössbauer data obtained at room temperature [28], according to which for low concentrations of Fe doping into the 4 Mn sites (Mn1 (12i), Mn2 (8 h), Mn3 (6f), and Mn4 (2d); using the hexagonal space group P6 3 / mcm), the Fe atoms were mainly localized in Mn2 (8 h) and Mn3 (6f) sites.…”

“…If the distortion is small enough, it can be expected that the crystal acquires this more symmetric configuration at a higher temperature. At high temperatures, hexagonal symmetry with space group P6 3 /mcm is indeed observed [17,28], while as the temperature decreases, a distortion occurs and the structure becomes orthorhombic, with space group Pnma, i.e. a subgroup of P6 3 /mcm.…”

“…However a high resolution study by Rasch et al [16] using synchrotron data demonstrated instead an orthorhombic symmetry. In another study, Volkov et al [17] and Kimber [28] also reported an orthorhombic Pnma structure for PMO at 298 K, albeit noting that it was close to a structural phase transition, with a pseudo-hexagonal cell at 298 K, evolving into an hexagonal structure at higher temperatures [17,28]. Stabilization of hexagonal structure at room-temperature after thermal treatment is not possible [17].…”

Синтез, структурные, магнитные и теплофизические свойства Pb-=SUB=-3-=/SUB=-Mn-=SUB=-7-=/SUB=-O-=SUB=-15-=/SUB=-, допированного ионами титана

“…Наиболее изучены соединения с перовскитоподобной структурой, однако существуют и другие семейства, в которых марганец встречается в разновалентных состояниях. Одним из таких соединений является Pb 3 Mn 7 O 15 [1][2][3][4]. Марганец входит в состав этого соединения в двух валентных состояниях: 3+ и 4+ в соотношении 4 : 3.…”

Analysis of the exchange magnetic structure in Pb3Mn7O15

“…Позже было обнаружено, что при нагревании Pb 3 Mn 7 O 15 ромбическая структура пространственной группы P nma, которая имеет место при комнатной температуре, трансформируется сначала в пространственно-модулированную структуру (при T s1 = 400 К), а затем (при T s2 = 560 К) в гексагональную структуру с пространственной группой P 6 3 /mcm [6]. В работе [7] показано, что структурный переход из гексагональной в ромбическую фазу при понижении происходит благодаря орбитальному и зарядовому упорядочению.…”