Crystal structure and spin-trimer magnetism of Rb 2.3 (H 2 O) 0.8 Mn 3 [B 4 P 6 O 24 (O,OH) 2 ] †

The novel borophosphate Rb 2.3 (H 2 O) 0.8 Mn 3 [B 4 P 6 O 24 (O,OH) 2 ] was prepared under hydrothermal conditions at 553 K. Its crystal structure was determined using single-crystal X-ray di ﬀ raction data obtained from a non-merohedral twin and re ﬁ ned against F 2 to R = 0.057. The compound crystallizes in the orthorhombic space group Pbcn , with unit-cell parameters a = 20.076(2) Å, b = 9.151(1) Å, c = 12.257(1) Å, V = 2251.8(2) Å 3 , and Z = 4. The title compound is the ﬁ rst example of a borophosphate with manganese ions adopting both octahedral and tetrahedral coordinations. Its unique crystal structure is formed by borophosphate slabs and chains of Mn 2+ -centered polyhedra sharing edges and vertices. These 2D and 1D fragments interconnect into a framework with open channels that accommodate Rb + cations and water molecules. Topological relationships between borophosphates built from three-membered rings of two borate and one phosphate tetrahedra sharing oxygen vertices, amended by additional PO 4 and HPO 4 tetrahedra, are discussed. The temperature dependence of the magnetic susceptibility of Rb 2.3 (H 2 O) 0.8 Mn 3 [B 4 P 6 O 24 (O,OH) 2 ] reveals predominant antiferromagnetic exchange interactions and the high-temperature e ﬀ ective magnetic moment corresponding to the high-spin S = 5/2 state of Mn 2+ ions. At 12.5 K, a magnetic transition is evidenced by ac-susceptibility and speci ﬁ c heat measurements. A spin-trimer model with the leading exchange interaction J ∼ 3.2 K is derived from density-functional band-structure calculations and accounts for all experimental observations.


Introduction
The chemical class of borophosphates comprises compounds with crystal structures built by PO 4 and BO 4 /BO 3 oxocomplexes sharing oxygen vertices. Similar to silicates, complex borophosphate anions may be zero-, one-, two-and three-dimensional oligomer units forming chains, layers or frameworks. 1 Intensive exploration of borophosphates in the last two decades is due to their fascinating structural chemistry and numerous technological applications. 2 activity as a cathode material for Li-and Na-ion batteries, 4 while Sn x (Ca 0.05 B 0.975 P 0.975 O 3.95 ) 1−x /C composites can be used as anode materials for Li-ion batteries. 5 Borophosphates may also have interesting connections to abiogenesis. According to ref. 6, some prebiological episodes could occur in environments with borophosphates, which arguably took part in special reactions in the form of hydrogels. Their derived chiral minerals served as drivers for prebiotic processes, thus providing conditions for the appearance of protocells.
Borophosphates with open microporous structures often comprise metal oxocomplexes and, therefore, not only exhibit adsorption, catalytic, and ion-conductive properties typical of zeolite-like compounds, but also demonstrate promising physical characteristics which depend on the nature of constituent metal atoms. The incorporation of transition metals into the borophosphate framework often leads to interesting features of forming phases. 7,8 Particularly, borophosphates with Mn 2+ -cations are of special interest not only with respect to catalysis applications, but also as potential materials with luminescence and magnetic properties  4 ) crystals with 16 ring pore openings show antiferromagnetic interactions between Mn 2+ ions. 9 KMnBP 2 O 7 (OH) 2 demonstrates long-range antiferromagnetic ordering and exhibits bright orange luminescence at room temperature. 10 Another Mn 2+ borophosphate, (NH 4 ) 6 [Mn 3 B 6 P 9 O 36 (OH) 3 ]·4H 2 O, reveals canted antiferromagnetic order at low temperatures. 11 In our search for functional crystals with transition metals and mixed anionic arrangement, a novel water-bearing rubidium manganese borophosphate was obtained under hydrothermal conditions. The new compound is characterized by a unique combination of six-fold and four-fold-coordinated Mn 2+ cations, not mentioned before to our knowledge. We present here its crystal structure and magnetic properties in comparison with related compounds featuring borophosphate slabs of similar topology.

Synthesis and structure determination
Colourless transparent needle crystals of the new phase with a maximum length of 200 μm ( Fig. 1) were prepared hydrothermally in the Rb 2 CO 3 -MnCl 2 -H 3 PO 4 -B 2 O 3 system. A mixture of these components at the 1 : 2 : 2 : 4 weight ratio was placed into a 4 ml stainless steel bomb with distilled water filling 80% of the volume. The experiment was con-ducted at a temperature of 553 K and a pressure of 70 bar over a period of 18 days followed by cooling of the furnace to room temperature. The reaction products in the form of druses of needle crystals were washed with water and dried. Their phase purity was confirmed by the agreement between the experimental powder X-ray diffraction pattern and the simulated diagram based on the single-crystal structural data.
A suitable single crystal of the title compound was analyzed with a scanning electron microscope (SEM) ‡ JEOL SEM (JSM-6480LV) equipped with an INCA Energy-350 energy dispersive (EDS) detector and an INCAWave-500 four-crystal wavelength dispersive spectrometer (WDS). The measurements were made at 20 kV and 7 nA, and the sample was stable under these conditions. X-ray spectral analysis provided a semi quantitative result with the Rb : Mn : P : B : O ratio close to 1 : 1.5 : 3 : 2 : 15, which is consistent with the results of our X-ray diffraction structural study.
The single crystal X-ray diffraction data were collected at ambient temperature by using graphite-monochromated Mo-Kα radiation with an Xcalibur-S area detector diffractometer. The intensities were corrected for Lorentz and polarization effects, and a numerical absorption correction based on Gaussian integration over a multifaceted crystal model was applied. An analysis of the experimental set of X-ray reflections with CrysAlisPro 12 has shown that the studied crystal was a non-merohedral twin with a twinning angle of about 8.5°. The reflections from different components of the sample were separated in the new set. 12% of reflections with partial overlap, for which individual contributions could not be revealed, were removed. All calculations were performed in the WinGX32 software package. 13 Atomic scattering factors and anomalous dispersion corrections were taken from the International Tables for Crystallography. 14 The crystal structure was solved via direct methods in the space group Pbcn and refined against the F 2 data with SHELX programs. 15 The final refinement was performed on the basis of all experimental intensities marked for each component in the HKLF5 reflection file, to the R factor of 0.057 (for 1891 unique reflections with I > 2σ(I)) with anisotropic displacement parameters for all non-hydrogen atoms. The positions of one independent H atom forming the hydroxyl group were obtained by difference-Fourier techniques and refined in an isotropic approximation. The O-H bond length was fixed by hard restraints to an empirical value of 0.85 Å in order to obtain comparable H-bond geometry not affected by arbitrary scatter of the refined O-H distance. The crystallographic characteristics of the new phase, the experimental conditions of the data collection, and the final results of the structure refinement are shown in Table 1. Table S1 † presents the atomic positions and equivalent isotropic displacement para-meters. § Characteristic interatomic distances are given in Table 2. A bond-valence calculation ( were investigated in the temperature range 2-300 K using both ac-and dc-magnetic susceptibility options of the "Quantum Design" Physical Properties Measurements System PPMS-9T. The magnetization of the title compound was measured up to 9 T in a static magnetic field and up to 30 T in a pulsed magnetic field at low temperatures. The specific heat of the pressed pellet of the sample was measured in the range 2-200 K by the relaxation method.

Description of the crystal structure and discussion
The asymmetric unit of the structure (    (16) 1.87 (7) 2.566 (15) 139 (10)  additional O atoms are situated at longer distances of 2.940(6) Å (Fig. 3). The tetrahedral coordination is relatively uncommon for Mn 2+ , although it has been reported for the KMnPO 4 and RbMnPO 4 crystal structures. 18,19 The patterns of the P-, B-and Mn-polyhedra distortion are consistent with the bond-valence calculation (  (Fig. 4). A symmetrically independent fragment of the borophosphate anion presents a three-membered ring of two borate and one phosphate tetrahedra sharing oxygen vertices, amended by additional PO 4 and HPO 4 tetrahedra (Fig. 5a). These basic building units are reproduced by symmetry elements of the Pbcn space group and form anionic borophosphate slabs (Fig. 5b), which interchange along the a axis with the chains of Mn polyhedra (Fig. 6) (Fig. 7).
Topologically identical slabs built by phosphate and borate tetrahedra have been previously described in several other crystal structures, which are listed in Table 3. The first such cobalt borophosphate with organic molecules filling structural channels was reported by Sevov. 20 In the (C 2 H 10 N 2 )Co [B 2 P 3 O 12 (OH)] crystal structure, the borophosphate slabs with three-and nine-membered rings share oxygen vertices with

Co-centered octahedra to form a 3D open framework.
Well-ordered protonated ethylenediamine molecules are fixed within the channels by hydrogen bonds. Later on, a series of isotypic orthorhombic compounds (C 2 H 10 N 2 ) M 2+ [B 2 P 3 O 12 (OH)] with M = Mg, Mn, Fe, Ni, Cu, and Zn was hydrothermally synthesized and studied using single-crystal X-ray diffraction (for the Mg phase) or by Rietveld-methods. 21 The Cd borophosphate, (C 2 H 10 N 2 )Cd[B 2 P 3 O 12 (OH)] (Fig. 8a), crystallizes in the same structure type. 22 All these isotypic compounds reveal different unit cell volumes that obviously depend on the size of the M 2+ cation (  dimensional inorganic fragments of the same borophosphate slabs with adjacent CdO 5 Cl octahedra, separated by organic templates of diethylenetriamine (Fig. 8b). 22 Accordingly, the c parameter of the (C 4 H 16 N 3 )[CdClB 2 P 3 O 12 (OH)] unit cell is changed significantly, and the inorganic structural fragments and organic DETA molecules alternate in the structure (Table 3). Isostructural borophosphates, all having the same D 15 2h = Pbca space group, may be obtained without organic templates. Two isotypic compounds, |K 2 (H 2 O)|[MB 2 P 3 O 12 (OH)] [M = Co or Ni], have been synthesized with K + ions instead of ethylenediamine as the organic template. They are constructed by the connection of tetrahedral layers and MO 6 octahedra, giving rise to a 3D framework with 8-ring channels aligned in the [010] direction. The negative charge of the framework is compensated by the K + ions located in the 8-ring channels together with water molecules. 23 Two more compounds, A + 2 Co 3 (H 2 O) 2 [B 4 P 6 O 24 (OH) 2 ] with alkaline ions, which play a directing role in the structure formation, were formed under hydrothermal conditions. 24,25 Their isotypic crystal structures include trimeric units of CoO 6 and CoO 4 (H 2 O) 2 octahedra sharing edges that further interconnect with the borophosphate slabs into a framework (Fig. 9). Large low-charged Rb + or Cs + ions occupy positions in open channels. Both orthorhombic phases are described by the same D 15 2h = Pbca space group and similar unit cell parameters, which are naturally larger in the case of Cs 2 Co 3 (H 2 O) 2  building blocks of three-membered rings of two borate and one phosphate tetrahedra features tetragonal symmetry (space group I4 1 /a). 26 The CoO 6 octahedra are introduced into the borophosphate network to form a complex open framework with a three-dimensional intersecting channel system. Its voids are populated by ammonium and diprotonated piperazine ions and water molecules.

Magnetic properties
The dc-magnetic susceptibility χ in a wide temperature range follows the modified Curie-Weiss law with the temperature independent term where χ 0 = −4 × 10 −4 emu mol −1 , the Curie constant C = 13.2 emu K mol −1 , and the Weiss temperature Θ = −16 K (Fig. S1 †). The negative value of χ 0 originates from diamagnetic contributions of individual ions. The absolute value of this contribution is in quite good agreement with the sum of Pascal's constants yielding −0.427 × 10 −3 emu mol −1 . 27 The Curie constant corresponds to the value of 8C = ng 2 S(S + 1) = 13.125 emu K mol −1 expected for n = 3 Mn 2+ ions in the chemical formula with the g-factor g = 2 and spin S = 5/2. The negative value of the Weiss temperature indicates the predominance of antiferromagnetic exchange interactions. At low temperatures,  The ac-magnetic susceptibility is in good agreement with the dc-data and reveals weak anomaly at T N = 12.5 K, as shown in Fig. 10. This anomaly may signal long-range magnetic ordering in the system. In contrast to conventional antiferromagnets, the magnetic susceptibility increases below the transition and displays a paramagnetic-like behavior at low temperatures. Further evidence of the magnetic transition is provided by the specific heat measurements shown in Fig. 11. A rather smeared anomaly is seen at T N = 12.5 K.
The field dependence of the magnetization is shown in Fig. 12. At 2.5 K, magnetization approaches the saturation value of 15μ B /f.u. around 27 T suggesting that a rather high magnetic field is required to overcome antiferromagnetic interactions between the Mn 2+ spins. Additionally, deviations from linear behavior are seen around 4 T.

Magnetic model
Individual magnetic couplings in Rb 2.3 (H 2 O) 0.8 Mn 3 [B 4 P 6 O 24 (O, OH) 2 ] were obtained from total energies of collinear spin configurations calculated using the projected augmented wave formalism implemented in the VASP code. 28 For density-functional theory (DFT) band-structure calculations, the Perdew-Burke-Ernzerhof flavor of the exchange-correlation potential 29 was chosen. Strong correlations in the Mn 3d shell were taken into account of the mean-field DFT+U level 30 with the on-site Coulomb repulsion U = 7 eV and Hund's exchange J = 1 eV. 31 Each magnetic coupling was derived from the total energies of four collinear spin configurations, as further explained in ref. 32. We used the experimental crystal structure that was modified as follows: (i) the Rb1 position was 100% occupied, while the sparsely occupied Rb2 and Rb3 positions were excluded; (ii) the O14 and O15 positions (disordered water molecules) were excluded too. These modifications change the overall composition to Rb 2 Mn 3 [B 4 P 6 (OH) 2 2 ], three nearest-neighbor couplings are J 1 = 0.14 K (3.478 Å, Mn1-Mn1), J 2 = 3.4 K (3.632 Å, Mn1-Mn2), and J 3 = −0.01 K (4.065 Å, Mn1-Mn2), where numbers in brackets stand for Mn-Mn distances (Fig. 13). The couplings beyond the nearest neighbors are below 0.02 K.
The difference between the nearest-neighbor couplings J 1 and J 2 is rooted in the different Mn-O-Mn angles, 103.74°and 115.59°, respectively. According to Goodenough-Kanamori-Anderson rules, larger Mn-O-Mn angles favor antiferromagnetic couplings, hence J 2 > J 1 . Despite its relatively short   Mn-Mn distance, the coupling J 3 is between the Mn1O 6 octahedron and Mn2O 4 tetrahedron that are not directly connected to each other. Therefore, no Mn-O-Mn superexchange pathway exists, and the coupling J 3 remains weak.
The coupling J 2 builds Mn1-Mn2-Mn1 trimers. The magnetic susceptibility of spin-5/2 trimers obtained from quantum Monte-Carlo simulations with J = 3.2 K, g = 1.98, and χ 0 = −4 × 10 −4 emu mol −1 perfectly reproduces experimental data down to 20 K (Fig. S1 †). This exchange coupling is in excellent agreement with the calculated J 2 = 3.4 K. The trimer model explains the increase in the susceptibility toward lower temperatures. Indeed, at low temperatures an individual trimer adopts the spin-5/2 state and behaves as a paramagnetic entity with the diverging susceptibility. Below 20 K, the magnetic transition at T N and ensuing deviations from the spin-trimer behavior are likely due to antiferromagnetic couplings between the trimers. However, these couplings are too weak for a reliable quantitative analysis.
For a spin-5/2 trimer with J = 3.2 K one expects saturation at H s ∼ 20 T. Experimentally, the saturation feature is very broad, and the saturation field cannot be determined with sufficient accuracy. Nevertheless, at 2.5 K and 20 T the magnetization reaches around 90% of the saturated value, which means that our estimate of J is in reasonable agreement with the magnetization data.  2 ] is a new borophosphate compound that reveals an original crystal structure. In contrast to other borophosphates with similar structural units, it shows an interesting combination of the magnetic high-spin Mn 2+ ions in both octahedral and tetrahedral coordinations. The ensuing magnetic interactions give rise to antiferromagnetic spin trimers that manifest themselves in the paramagnetic-like behavior, with the magnetic susceptibility increasing upon cooling. Weak interactions between these trimers induce a magnetic transition ( presumably, antiferromagnetic ordering) around 12.5 K.