“Ampoule in ampoule” design to grow single crystals of FeSe1−xSx.

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The bond-valence-sum method is used to calculate the hole distribution in the spin ladders of [M2Cu2O3] m [CuO2] n -type cuprates with m = n = 1 (where M are divalent or trivalent cations). It is shown that the appearance of superconductivity in this system is related to hole transfer from chain planes to the spin-ladder plane. The role of the valency and size of the M atoms is discussed in detail. § 1. Introduction Among various spin-ladder compounds, the [M2Cu2O3] m [CuO2] n cuprates (where M are divalent or trivalent cations) have attracted most attention because they consist of both simple chains and two-leg ladders of copper ions. This system consists of Cu 2 O 3 spin-ladder planes as well as CuO 2 chains. Four phases of this family (for m/ n ratios equal to 1/1, 5/7, 7/10 and 9/13) have been successfully synthesized and a superconducting phase has been shown to exist in the 1/1 (Leonyuk et al. 1998), 5/7 (Szymczak et al. 1999) and 7/10 (Uechara et al. 1996 phases.To understand the appearance of superconductivity in the [M2Cu2O3] m [CuO2] n system, it is crucial to clarify the distribution of the self-doped holes between the chain and ladder planes. It was suggested previously (Uechara et al. 1996, Szymczak et al. 1999) on the basis of experiments that superconductivity is related to the hole concentration transferred from the chain planes to the spin-ladder planes. In this letter we examine this problem by means of the bond-valence-sum (BVS) method.We decided to analyse the [M2Cu2O3] m [CuO2] n system with m = n = 1 because of the detailed positional atom parameters (Leonyuk et al. 1998) that are available for both superconducting (SC) and non-superconducting (NSC) single crystals. Similar BVS calculations for m = 7 and n = 10 system have recently been performed (Kato

The manganese orthophosphate, Mn(PO), is characterized by the rich variety of polymorphous modifications, α-, β'-, and γ-phases, crystallized in monoclinic P2/c (P2/n) space group type with unit cell volume ratios of 2:6:1. The crystal structures of these phases are constituted by three-dimensional framework of corner- and edge-sharing [MnO] and [MnO] polyhedra strengthened by [PO] tetrahedra. All compounds experience long-range antiferromagnetic order at Neel temperature T = 21.9 K (α-phase), 12.3 K (β'-phase), and 13.3 K (γ-phase). Additionally, second magnetic phase transition takes place at T* = 10.3 K in β'-phase. The magnetization curves of α- and β'-modifications evidence spin-floplike features at B = 1.9 and 3.7 T, while the γ-Mn(PO) stands out for an extended one-third magnetization plateau stabilized in the range of magnetic field B = 7.5-23.5 T. The first-principles calculations define the main paths of superexchange interaction between Mn spins in these polymorphs. The spin model for α-phase is found to be characterized by collection of uniform and alternating chains, which are coupled in all three directions. The strongest magnetic exchange interaction in γ-phase emphasizes the trimer units, which make chains that are in turn weakly coupled to each other. The spin model of β'-phase turns out to be more complex compared to α- or γ-phase. It shows complex chain structures involving exchange interactions between Mn2 (Mn2', Mn2″) and Mn3 (Mn3', Mn3″). These chains interact through exchanges involving Mn1 (Mn1', Mn1″) spins.