New Developments in the Synthesis, Structure, and Applications of

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New developments in the synthesis, structure and applications of borophosphates and metalloborophosphates Anja-Verena Mudring, and Min Li Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b01035 • Publication Date (Web): 30 Oct 2015 Downloaded from http://pubs.acs.org on November 1, 2015

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Crystal Growth & Design

New developments in the synthesis, structure and applications of borophosphates and metalloborophosphates Min Lia and Anja-Verena-Mudringa,b,c* a b

Department of Chemistry, Iowa State University, 1605 Gilman Hall, Ames, IA, 50011, USA

Department of Materials Science and Engineering, Iowa State University, 2220 Hoover Hall, Ames, IA, 50011, USA c

Critical Materials Institute, Ames Laboratory, 333 Spedding Hall, Ames, IA, 50011, USA [email protected], Tel:+1 515 5095616

Abstract An overview about the recent key developments of borophosphate chemistry since 2007 is given. The structural chemistry (B:P ratio, fundamental building units (FBUs), dimensionality and metal coordination), possible physical, optical and chemical properties are discussed in detail in terms of materials obtained by traditional solid-state reactions, flux methods, hydrothermal and ionothermal reactions. Borophosphates (BPOs) exhibit a tremendous structural variety. Which structure is formed depends critically on the chosen starting materials and synthetic conditions. For example, for metalloborophosphates (MBPOs) changing the metal-precursor can result in the formation of a new BPO. MBPOs containing chains or extended networks of interconnected transition metal-oxide polyhedra exhibit remarkable magnetic coupling schemes and electronic behavior aside from interesting optical behavior and catalytic properties. The exploration of novel fundamental building units (FBUs), such as FBUs with P-O-P bonds and 2D mixed-coordinated anionic partial structures that have not been observed previously advocates the great potential in designing novel functional BPOs for advanced applications. Whilst many BPOs initially obtained from conventional synthetic methods can be prepared by hydrothermal synthesis, it has recently be realized that BPOs obtained by ionothermal methods often could not be synthesized by conventional synthetic methods. Thus, novel synthetic approaches offer access to new materials.

1. Introduction Crystalline inorganic open-framework solids including conventional zeolites, alumosilicates, and related porous materials are of scientific and technological interest in applications ranging from microelectronics to medical diagnosis 1. Examples include AlPOs (Al-P-O systems) 2-4, GaPOs (Ga-P-O systems) 5-7, MeAPOs (metal-Al-P-O systems) 8-10, and analogues. They all have in common that complex

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oxo-anions are interlinked via oxygen atom bridges which gives rise to open-framework structures. The broad interest in this class of compounds arises from the fact that framework structures similar to zeolites are capable of interacting with atoms, ions and molecules not only at their surfaces, but throughout the bulk of the material. Astonishingly, framework structures of borophosphates and related Me-B-P-O systems have received so far comparatively little attention, although BPO4 itself is applied industrially as a catalyst for hydration, dehydration, alkylation and oligomerization reactions and has been extensively investigated for that reason 11. Combining oxygen-interlinked borate and phosphate anions bears a tremendous potential for new structural arrangements and, so far, after a pioneering study in 199412, a broad spectrum of anion partial structures featuring oligomeric units 13-15, chains 16-18, ribbons 19-21, layers 22-24 and three-dimensional frameworks 25-27 have been reported. The construction set of anions (Bψ3,Bψ4 and Pψ4, non-protonated and protonated (ψ=O, OH)) not only possess multifaceted linkages, the (pseudo-)tetrahedral building blocks may also give rise to non-centrosymmetric and chiral structures even without chiral starting materials 28. This provides a great deal of motivation for exploring new nonlinear optical (NLO) materials 29-31 The potential applications for borophosphates can further be enhanced by incorporation of metal cations endowing the materials with additional interesting physical properties such as luminescence, magnetism and conductivity as well as enhanced catalytic properties. So far, about 25 different metal cations have successfully been incorporated into BPOs 32. This structural and compositional diversity, in combination with the potential properties, gives researchers a toolbox to design advanced functional inorganic solids.

2. Structural Aspects The complexity of borophosphate structures provides a strong impetus for extensive study of their crystal chemistry shedding light upon the delicate relationship between structures and properties. The first approach to systemize borophosphate structural chemistry appeared in 1997. 33 Where applicable, the classification principles developed for silicates by Liebau and Pauling were transferred to BPOs and BPOs were classified based on the linking principles of the primary building units. However, these rules had to be amended and supplemented owing to the larger structural complexity of this compound class. Consequently, BPOs are first divided into anhydrous and hydrated phases, followed by a classification according to the molar ratio of boron to phosphorus. Different complex oxoanions can be observed in BPOs. Hydrated BPOs with a B:P ratio ≤1 are exclusively built of borate and phosphate tetrahedra. That is, boron in 3-fold coordination is only present when B:P >1 (principle i). Terminal oxygen positions of BO3 groups are always protonated (principle ii). Three-dimensional borophosphate frameworks are rare (principle iii) and P-O-P linking is not observed (principle iv). The steady growth of BPO structures


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prompted Kniep et al. to develop an improved classification for BPOs based on the anionic compositions as well as connection patterns 34. Anionic arrangements were divided into “tetrahedral” BPOs in which boron and phosphorus are all tetrahedrally coordinated, while for the “mixed coordinated” BPOs, trigonal BO3 groups can be observed too. Metalloborophosphates additionally contain metal-tetrahedra units in the condensed macroanions. Over 30 fundamental building units (FBUs) have been identified on the basis of the coordination pattern, the B:P ratio and the dimensionality. A special representation nomenclature for FBUs was developed: Tetrahedral units are represented by □, trigonal planar units by ∆, for rings polyhedral denominators are enclosed by , central units in a branching unit by [...], attached branches are separated by ｜. When two units are connected via shared polyhedra the number of shared polyhydra is specified by -,=, ≡. The degree of branching B indicated by uB (unbranched), oB (open-branched), lB (loop-branched), cB (cyclo-branched), mixed branching by a combination of these denominators, for example olB for open- and loop-branched). Detailed analysis of the known structures revealed new building principles. For example, for metalloborophosphates (MBPOs), no connections between equal tetrahedral specimens are found in the anionic partial structures. A four-membered ring of two phosphate groups interconnected by a borate unit and an oxo-metallate tetrahedron is the dominant structural motif 35-37. The B:P ratio and the dimensionality of borophosphates are related, and it was found that the dimensionality of mixed-coordinated borophosphate anions is limited to one-dimension. Six different B:P ratios were found for tetrahedral (1:1,2:3,1:2,2:5,1:3,1:4), and eight for mix-coordinated BPOs (1:1,1:3,2:3,3:4,6:1,5:1,3:1,3:2,), respectively. The number of new BPO structure types is continuously growing with ongoing research activities and general building principles and structural patterns become more evident. An open-branched 12-membered ring was found in KMBP2O8 (M=Sr, Ba) 29, novel three-dimensional anion partial structures such as [H2B2P4O16]4- 38, [B2P3O14(OH)]8- 39 and [B6P9O36OH)3]12- 40 were discovered, and the illumination of the structure of Li2B3PO8 has extended the set of FBUs for mixed-coordinated BPOs to two-dimensions 41. Several BPOs where P-O-P linkages have been observed has broadened the connection patterns of BPOs beyond Pauling’s fourth rule 42. Along with the rapid development in practical applications of BPOs, the rise of novel FBUs demands urgently further investigation that covers the newest aspects of BPOs in synthetic techniques and structural chemistry if potential applications are to be better understood and exploited. Of special interest is elucidating the structural features that govern physical properties of BPOs. For example, a linear chain structure as observed in (MII=Mn, Fe, Co, Ni) 28,43 and BiM2BP2O10 (M=Co, Ni) 44 may give rise to single-chain magnets (SCMs) 45,46. If the MeBPOs structure exhibits chiral symmetry, the observation of a magnetochiral dichroic (MChD) effect can be envisaged 47.



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3. Synthesis 3.1. High-temperature solution growth and traditional solid-state synthesis High-temperature solution growth (HTSG) or traditional solid-state synthesis has been widely conducted to obtain temperature stable phases. Most of known BPOs synthesized by this way show no substantial weight loss until 900 oC 48-50. The formation of BPOs by conventional high-temperature synthesis typically involves the elimination of water molecules and organic templates, which favors the formation of dense, and often thermodynamically favored, phases that are unfortunately quite often not desired as practical materials. A summary of BPOs obtained by HTSG methods are listed in Table 1. Novel FBUs containing P-O-P connections will first be discussed in details, followed by Li-containing BPOs in terms of their electrical conductivities. Nonlinear optical (NLO) crystals built from three different FBUs will be investigated on the response of second-harmonic generation (SHG) measurements. At last, the magnetism of MBPOs originating from chains of interconnected complex metal oxide will be discussed. Table 1. BPOs prepared by high-temperature solution growth (HTSG) or solid-state reactions reported since 2007. a

b

ଷ ௗ ஶ[‫ܲܤ‬ଷ ∅ଵଵ ]

N

B2/4, P1/4,P2/4

T

50

ଵ ஶ {[‫ܲܤ‬ସ ∅ଵସ ]}ଶ

N

B2/4, B4/4 P1/4,P2/4

T

50

ଶ ஶ [‫ܤ‬ସ ଼ܲ ∅ଷ଴ ]

N

B3/4, P1/4, P2/4,P3/4

T

48

ଶ ஶ {[‫ܲܤ‬ସ ∅ଵସ ]}ଶ

N

B2/4, P1/4,P2/4

T

48

଴ ஶ [‫ܤ‬ଶ ܲହ ∅ଵଽ ]

N

B4/4, P1/4,P2/4

T

51

ଶ ஶ {[‫ܲܤ‬ଶ ∅଼ ]}ଶ

N

B3/4, P1/4,P2/4

T

48

P-1 (2)

଴ ஶ [‫ܲܤ‬ଶ ∅଼ ]

K

B4/4, P2/4

T

52

3:1

P-1 (2)

ଶ ஶ[‫ܤ‬ଷ ܲ ∅଼ ]

N

B2/3, P1/4,P2/4

M

41

KMBP2O8 (M=Sr, Ba)

1:2

I-42d (122)

ଷ ஶ {[‫ܲܤ‬ଶ ∅଼ ]}଺

N

B3/4, P1/4,P2/4

T

29

10

KPbBP2O8

1:2

I-42d (122)

ଷ ஶ {[‫ܲܤ‬ଶ ∅଼ ]}଺

N

B3/4, P1/4,P2/4

T

31

11

RbPbBP2O8

1:2

I-42d (122)

ଷ ஶ {[‫ܲܤ‬ଶ ∅଼ ]}଺

N

B3/4, P1/4,P2/4

T

49

No.

BPOs

B:P

SG

1

CsFe(BP3O11)

1:3

Pnma (62)

2

Cs2Cr3(BP4O14) (P4O13)

1:4

P21/c (14)

3

Li2Cs2B2P4O15

1:2

P-1 (2)

4

Li3M2BP4O14 (M=K, Rb)

1:4

Cmca (64)

5

K7B2P5O19

2:5

P-1 (2)

6

LiK2BP2O8

1:2

P21/n (13)

7

Li3BP2O8

1:2

8

Li2B3PO8

9

FBUs


N/K

Qn/CN

c

T/M

Ref.

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12

Na3Cd3BP4O16

1:4

Pmc21 (26)

଴ ஶ[‫ܲܤ‬ସ ∅ଵ଺ ]

K

B4/4, P1/4

T

30

13

MII[BPO5] (M=Ba, Sr)

1:1

P3221(154)

ଵ ஶ [‫∅ ܲܤ‬ହ ]

K

B4/4, P2/4

T

53

14

BiM2BP2O10 (M=Co, Ni)

1:2

P21/m (11)

଴ ஶ [‫ܲܤ‬ଶ ∅ଵ଴ ]

K

B2/4, P1/4

T

44

15

MIII2[BP3O12] (M=Fe, In)

1:3

P63/m (176)

଴ ஶ [‫ܲܤ‬ଷ ∅ଵଶ ]

K

B3/3, P1/4

M

54

16

Cr2[BP3O12]

1:3

P63/m (176)

଴ ஶ [‫ܲܤ‬ଷ ∅ଵଶ ]

K

B3/3, P1/4

M

55

“N” stands for new FBUs, “K” indicates known FBUs that have been described in the review by Kniep. et al. A star “*” means that the composition of the FBUs are the same as known ones, while the structural chemistry is slightly different; “Q” indicates the connectivity of boron and phosphorous and n is the coordination number (CN); “T” means that the only tetrahedral complex anions are present in the structure; “M” stands for mixed-coordinated anionic partial structures; ∅ may be O or OH.

The surprising discovery of P-O-P bonds in BPOs significantly expands the structure diversity since terminal PO4 tetrahedra can further condense into polyanionic groups allowing the dimensionality of the anionic partial structures not to depend on the B/P ratio. The first examples of BPOs with P-O-P connection are CsFe(BP3O11) and Cs2Cr3(BP4O14) (P4O13) 50. For CsFe(BP3O11), a borate tetrahedron branches by one PO4 group and one P2O7 group via shared oxygen corners yielding a FBU of composition [B(PO4)(P2O7)]4- (4□:4□, □ denotes a tetrahedral unit, for details on the nomenclature see ref 34). These FBUs are interlinked via common corners generating a 3D anion framework. In CsFe(BP3O11), a phosphate tetrahedron forms with two borate tetrahedra a 3-membered ring where each of the borate tetrahedra bear a P2O7 unit forming a FBU that can be summarized as [B(PO4)(P2O7)]4- (4□:4□). The FBUs are interlinked by corners to a 3D anion framework. Cs cations for charge balance are located in the empty space. In the structure of Cs2Cr3(BP4O14)(P4O13) 50, BO4 and P2O7 groups are connected to each other alternatively via B-O-P-O-P-O-B linking, leading to 1D anionic chains of [B(P2O7)2]n5n- that run along the c axis. CrO6 octahedra are further linked with [B(P2O7)2]n5n- and [P4O13]6- units to form two types of layers which are interconnected by P-O-Cr linking to give 3D [Cr3BP8O27]2- anions. In contrast to the 1D anionic partial structure discovered in Cs2Cr3(BP4O14)(P4O13), the FBUs of composition [B(P2O7)2]n5n- (5□:[□] 2□｜2□) in Li3M2BP4O14 (M=K, Rb) 48 are condensed through common vertexes to generate a complex 2D anionic layer perpendicular the b axis. Li2Cs2B2P4O15 also features 2D anionic layers, whereas the FBU [B4P8O30]8- (12□: ＜3□＞｜□ [＜4□＞] □｜＜3□＞, denotes a ring motif, for details of this nomenclature see ref. 34) is composed of one branched eight membered-ring [B2P4O16] (6□: □＜4□＞□) and two six-membered rings of [BP2O8] (3□: ＜3□＞) 48. The LiO4 tetrahedra are located in/between anionic layers by sharing O atoms with phosphate and borate groups, thereby giving rise to 3D framework. Cs+ cations reside in the network cages. Another example of P-O-P connection is found in K7B2P5O19 51 which has a unique 0D FBU [B2P5O19]7- (7□: ＜3□＞-＜3□＞-＜3□＞).



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All the known POP-connected FBUs, corresponding anionic partial structures and the frameworks are illustrated in the sequence of zero-dimensional (0D) to three-dimensional (3D) in terms of anion partial structures in Fig. 1. Common to all these FBUs is that boron and phosphorus atoms are all tetrahedrally coordinated with B/P ratio less than 1, and the longest P-O distances present are always involved in P-O-P connections. Examination of the preparation of pure polycrystalline samples suggests that starting materials containing similar P-O-P connections as in the final products might provide a route for the synthesis of POP connected BPOs.

Figure 1. Structural features of all known borophosphates exhibiting P-O-P connections in the sequence from zero-dimensional (0D) to three-dimensional (3D) anionic partial structures. Left: the fundamental building units (FBUs); Middle: the anionic partial structures; Right: the corresponding frameworks. (1) K7B2P5O19; (2) Cs2Cr3(BP4O14) (P4O13) ; (3) Li3M2BP4O14 (M=K, Rb) ; (4) Li2Cs2B2P4O15 ; (5) CsFe(BP3O11). Phosphate tetrahedra, purple; borate tetahedra, cherry; CrO6 octahedra, green; LiO4 tetrahedra, blue; FeO6 octahedra, parakeet; phosphorus atoms, pink; boron atoms, cherry red; cesium atoms, denim blue; lithium atoms, chiffon.


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Traditional research in BPOs has also focused on studying glass forming materials. Lithium borophosphate glasses are widely studied in terms of their conducting properties 56-58. Crystals of materials with a composition Li22B11P13O60 and Li2B3PO8 were observed in a study on borophosphate glass formation in the Li2O-B2O3-P2O5 system 59, although their structures were not analyzed in detail. Subsequent detailed studies revealed that Li22B11P13O60 was actually Li3BP2O8 (triclinic; space group P-1, a=5.1888(5) Å, b=7.4118(7) Å, c=7.6735(7) Å, α=101.179(3)o, β=105.067(3)o, γ=90.335(3)o) featuring one-dimensional [BP2O8]3chains (6□:□ ＜ 4□ ＞ □) along the c axis. Li2B3PO8 is the first example of a mixed-coordinated borophosphate exhibiting two-dimensional sheets of [B3PO8]2formed by sharing oxygen atoms in the terminal trigonal borate groups of FBUs (4△4□: ＜ 2△□ ＞ - ＜ 4□ ＞ - ＜ □2△ ＞ ). The ion conductivities measured for the polycrystalline samples were 1.5 × 10-5 S cm-1 and 1.6 × 10-7 S cm-1 at 583 K for Li3BP2O8 and Li2B3PO8, respectively. The different conductivities have been related to the structural differences of both materials, Fig. 2. In Li3BP2O8, the two crystallographically distinct Li sites with Li-Li distances from 2.531(5) Å to 2.624(7) Å exhibit zigzag chains along the b axis. Due to the relatively short Li-Li distances between the zigzag chains (3.688(7) Å to 3.774(7) Å), a diffusion path can be anticipated in the a c direction. In contrast, in Li2B3PO8, the Li+ ions are located in the rings of the well separated two-dimensional sheets, resulting in the lack of short Li-Li contacts.

Figure 2. The open-framework structures of Li2B3PO8 and Li3BP2O8. PO4 tetrahedra in pink; BO4 tetrahedra in cherry.

The class of borates and phosphates comprises also well-known second-order nonlinear optical (NLO) materials such as β-BaB2O4 (BBO) 60, LiB3O5 (LBO) 61, KTiOPO4 (KTP) 62 and KH2PO4 (KDP) 63. The rational combination of these two groups in one structure is anticipated to be an effective way to realize new NLO materials. Additionally, the introduction of transition metal ions susceptible to second-order Jahn-Teller distortions (d0 metal ions) and cations with non-bonded electron pairs (Se4+, Te4+ or Sb3+, etc.) might also support SHG generation 64-66. In this context, metal borophosphates become promising NLO materials.



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HTSG leads to the successful synthesis of MIMIIBP2O8 (MI=K, Rb; MII=Sr, Ba, Rb) crystallizing with the non-centrosymmetric space group I-42d I

II

29

. The FBUs of

I

open-branched 12-memerbered rings in M M BP2O8 (M =K, Rb; MII=Sr, Ba, Rb) (18□: □｜□｜□＜12□＞□｜□｜□) are interconnected by borate groups to form a three-dimensional anionic framework. The alkali metal cations (K/Rb) are eightfold coordinated by oxygen atoms, where six oxygen atoms are connected with five phosphate groups (four in a unidentate way and one through edge-sharing oxygen atoms), and the other three groups (two borates units through corner-sharing and one phosphate through edge sharing oxygen atoms). The solids containing Pb2+ cations show significantly higher SHG coefficients than that of without Pb2+. Calculations of local dipole moments indicate that the K/Pb polyhedra give the main contribution to the SHG effect, and the lone pairs on the Pb2+ cations are stereoactive, which enhance the SHG effect. Na3Cd3BP4O16 crystallizes in the orthorhombic space group Pmc21 with zero-dimensional FBUs of composition [B(PO4)4]9- (5□:[□]□｜□｜□｜□), which is an example for an “propeller-like” oligomeric anionic unit. 30 Cd atoms are connected with six oxygen atoms to form distorted CdO6 octahedra, while the cations residing in the mixed Na/Cd sites are surrounded by oxygen atoms to give Na/CdOn (n=6, 7, 8) polyhedra with coordination numbers ranging from 6 to 8. Two polyhedra with mixed metal occupancy alternate with CdO6 octahedra giving rise to one-dimensional chains along the c axis. These chains align parallel the [010] direction through sharing edges and vertexes to planes. The planes are connected to each other generating a 3D framework with tunnels along the b axis, in which the oligomeric borophosphate units are located. UV-vis-NIR diffuse-reflectance spectroscopy shows that Na3Cd3BP4O16 possesses a wide transparent region with the cutoff edge at about 360nm. SHG measurement shows that the material exhibits a SHG effect 1.1 times larger than that of KDP. The positive direction along the c axis for the total direction of the unit cell polarization suggests that the SHG effect may be ascribed to the special arrangement of BO4, PO4, CdO6, and Na/CdOn (n=6, 7, 8) polyhedra. Table 2. Atomic coordinates and equivalent isotropic displacement parameters for MIMIIBP2O8 (MI=K, Rb; MII=Sr, Ba, Rb) and Na3Cd3BP4O16. (KDP=KH2PO4) and SHG effect. Magnitude 10-4 esu Compounds

y (b)

z (c )

Debye

cm A-3

Species

x (a)

KBaBP2O8

BaO8

-2.834

0.1783

0

2.84

-

1/3 KDP

KSrBP2O8

SrO8

-1.696

-0.1557

0

1.70

-

1/5 KDP

RbPbBP2O8

Rb/PbO8

-4.056

-5.987

-1.936

7.48

-

~1 KDP

Pb(1)O5

-0.0045

-5.5552

1.6791

5.8034

279

Pb4O(BO3)2

Pb(2)O5

-3.3453

3.6766

2.9906

5.8011

279

Pb(3)O4

6.8188

0.0536

4.9004

8.3972

404


SHG effect

~ 3 KDP

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Pb(4)O3

2.6710

-0.8839

-8.9321

9.3647

451 10-4 esu

Species

KPbBP2O8

x (a)

y (b)

z (c )

Debye

cm A-3

K/PbO8

-3.3453

3.6766

2.9906

5.8011

117

B(1)O4

1.0767

0.5589

-1.2475

1.8851

76

P(1)O4

0.5809

-0.4239

-1.6524

1.8021

37

-10.7964

-24.6302

-4.6722

27.2954

Total

SHG effect

1 KDP

164 10-4 esu

Species

Na3Cd3B(PO4)4

x (a)

y (b)

cm A-3

z (c )

Debye

-10.0811

10.0811

303

BO4

0.0000

0.0000

Na/CdOn

0.0000

0.0000

14.547

14.547

437

PO4

0.0000

0.0000

21.9137

21.9137

659

CdO6

0.0000

0.0000

3.5288

3.5288

106

Total

0.0000

0.0000

0.8144

50.071

1505

SHG effect

1.1 KDP

High-quality optical crystals of two isotypic borophosphates KMBP2O8 (M=Sr, Ba) have been prepared using a top-seeded solution growth method. 29 Their SHG coefficients are measured to be 0.57 and 0.43 times that of KDP. Theoretical calculations carried out to understand the materials’ optical behavior indicate that: (i) in BaBPO5, the p-electron orbital of oxygen in PO4 plays a counteractive role, while the Ba-5d orbital and the p-orbital of oxygen in BO4 groups exhibit a positive contribution to the SHG effect; (ii) In SrBPO5, the NLO-active units are almost entirely BO4 groups. This is attributed to the absence of a d-orbital on the Sr atom, which results in a relative increase of the density of states (DOS) at the boron and phosphorus atoms. This change of DOS decreases the contribution of metal cations and increases the contribution of BO4 groups; (iii) the disappearance of the counteractive contribution of PO4 groups in SrBPO5 may result from weak interactions between PO4 and metal cations. The electronic structure and partial DOS suggest that the d-orbitals of Ba cations are dominant in the lowest unoccupied molecular orbitals (LUMO) for BaBPO5, while B and P in SrBPO5 are dominant in these LUMO. Therefore, Ba-5d orbital is closely related to the properties of PO4 groups. Regardless of the extensive study in anionic partial structures of MBPOs, the connection patterns of cations, especially for the transition metal ions, also play a crucial role in determining the structural architectures as well as potential properties such as stunning magnetic behavior 67-69. For instance, inorganic materials featuring transition-metal oxides with a linear chain structure are of particular interest in solid-state physics and chemistry offer the possibility of studying single-chain magnets (SCMs) 46. One representative example for such a material in the family of



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MBPOs is BiM2BP2O10 (M=Co, Ni) which contains quasi-1D linear chains built of CoO6 octahedra sharing common edges (Fig.3), 44 which both feature antiferromagnetic exchange interactions between transition metal ions. Yet, the magnetization as a function of magnetic field for BiCo2BP2O10 and for BiNi2BP2O10 at low temperature is quite different. For the Ni-containing material, magnetization isotherms obtained at 2K follow a linear variation in the overall range of the magnetic field strength (0-80 kOe), and the high value of magnetization attained indicates that BiNi2BP2O10 is featuring a canted antiferromagnetic interaction. For the Co-compound, a hysteresis loop with a large coercive field (H=48-140 kOe) is observed. The magnetic susceptibilities measured at various applied fields over a temperature range of 2 to 50K reveal this field-induced metamagnetic transition from a antiferromagnetic to ferromagnetic ground state. As studies on the metal linear chain system reveal uncommon magnetic exchange interactions, low-dimensional magnetic MPOs materials are of special interest.

Figure 3. (a) View of the structure of BiCo2BP2O10 along the a-axis. (b) 1/χ vs T and χ vs T plots for BiCo2BP2O10; (c) 1/χ vs T and χ vs T plots for BiNi2BP2O10; the dark red line represents the linear fit of data according to the Curie–Weiss law. (d) Magnetic susceptibilities of BiCo2BP2O10 at different applied fields (H = 5, 20, 30, 50, 60, 80 kOe) over a temperature range of 2–50 K. ((b),(c), and (d) are taken from ref.44.)


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The structure of Cr2[BP3O12] features Cr2O9 dimers of two face sharing CrO6 units interconnected by B(PO4)3 groups as shown in Fig. 4. Its magnetic behavior was extensively studied using magnetic susceptibility, neutron diffraction, and electron spin resonance (ESR) measurements 55. The Weiss temperature θ =139K, and the broad maximum around Tmax=85K (~61% of θ) indicate sizable antiferromagnetic (AFM) spin correlation. The experimentally determined effective magnetic moment of µeff =3.987 µB/Cr is slightly larger than the spin-only value µeff = 3.88 µB/Cr, indicating that the orbital moment is completely quenched. Remarkly, Cr2[BP3O12] entails the spin lattice of alternating chains made from intradimer coupling J1 and interdimer coupling J1’. In contrast to other related compounds, the intradimer coupling J1 exceeds the interdimer coupling J1’, and exhibits distinct J1’/J1~0.5. The critical point with the gapless ground state is found at J1’/J1~0.41. This critical point is one of the reasons for the long-range AFM ordering. The nearest-neighbor spins within a chain are antiparallel, in accord with the AFM nature of J1’and J1. Moreover, the large number of interchain couplings per magnetic site in Cr2[BP3O12] (the coordination number of interchain amounts to six and three for Jic1 and Ji2, respectively) may also reduce quantum fluctuations. The AFM coupling found for Fe2[BP3O12] suggests that this compound features a similar spin lattice in a more classical regime of a spin-5/2 system. 54

Figure 4. (i) Crystal structure of Cr2[BP3O12] viewed

along [0001]. (ii) View perpendicular to [0001]. The

pathways of the interchain couplings Jic1 and Jic2 are indicated by dark gray (dark blue) and light gray (red) lines, respectively. Thick arrows denote the experimental magnetic structure refined in Γ1. (iii) The intradimer



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coupling J1 (double line) as well as the interdimer coupling J′1 (single line) are shown. (iv) Experimental magnetic susceptibility χ(T) of Cr2[BP3O12] measured at 5 T (circles) and fit with the model of coupled bond-alternating S=3/2 chains (line). (v) Curie-Weiss fit to experimental χ(T). (vi) Field dependence of χ(T). Note the long-range AFM ordering transition at TN=28 K. (All figures are taken from Ref.55.)

3.2. Hydrothermal synthesis Since the first zeolite-like metal borophosphate (C2H10N2)[CoB2P3O12(OH)] was hydrothermally prepared by Sevov in 1996, 11 great efforts have been dedicated to the synthesis of BPOs using hydrothermal conditions. To obtain MBPOs under such conditions, sources for the framework forming complex anion (borate and phosphate), cations for charge balance and the water, are filled in an autoclave, sealed and heated under autogenous pressure to temperatures

New Developments in the Synthesis, Structure, and Applications of

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