Hexagonal ferrite

{{Short description|Hexaferrite}}

'''Hexagonal ferrites''' or '''hexaferrites''' are a family of ferrites with hexagonal crystal structure. The most common member is BaFe₁₂O₁₉, also called barium ferrite, BaM, etc. BaM is a strong room-temperature ferrimagnetic material with high anisotropy along the ''c'' axis.<ref>{{Cite journal |last=Pullar |first=Robert C. |date=2012-09-01 |title=Hexagonal ferrites: A review of the synthesis, properties and applications of hexaferrite ceramics |url=https://www.sciencedirect.com/science/article/pii/S0079642512000369 |journal=Progress in Materials Science |language=en |volume=57 |issue=7 |pages=1191–1334 |doi=10.1016/j.pmatsci.2012.04.001 |issn=0079-6425|url-access=subscription }}</ref>

All the hexaferrite members are constructed by stacking a few building blocks in a certain order.

== Basic building blocks ==

left|thumb|221x221px|S block structure extracted from an M-type ferrite. left|thumb|220x220px|The A layer of S block composed of Me^S-centered tetrahedron and Me^S-centered octahedron. thumb|R block structure extracted from an M-type ferrite. thumb|R block structure, view along -''c'' ferrite direction. left|thumb|The A layer of S block, view along -''c'' ferrite direction. left|thumb|220x220px|The B layer of S block made up of edge-sharing Me^S-centered octahedron. thumb|T block structure extracted from a Y-type ferrite. thumb|T block structure, view along -''c'' ferrite direction. left|thumb|The B layer of S block, view along -''c'' ferrite direction.

=== S block === The S block is very common in hexaferrites, which has a chemical formula of Me^S₆O₈²⁺. Me^S are smaller metal cations, for example, Fe and other transition metals<ref>{{Cite journal |last1=Ishiwata |first1=Shintaro |last2=Terasaki |first2=Ichiro |last3=Azuma |first3=Masaki |last4=Takano |first4=Mikio |date=2008-05-01 |title=High pressure synthesis and structure of a new magnetoplumbite-type cobalt oxide SrCo₁₂O₁₉ |url=https://www.sciencedirect.com/science/article/pii/S0022459608001060 |journal=Journal of Solid State Chemistry |language=en |volume=181 |issue=5 |pages=1273–1278 |doi=10.1016/j.jssc.2008.02.024 |bibcode=2008JSSCh.181.1273I |issn=0022-4596|url-access=subscription }}</ref> or noble metals.<ref>{{Cite journal |last1=Foo |first1=M. L. |last2=Huang |first2=Q. |last3=Lynn |first3=J. W. |last4=Lee |first4=Wei-Li |last5=Klimczuk |first5=Tomasz |last6=Hagemann |first6=I. S. |last7=Ong |first7=N. P. |last8=Cava |first8=R. J. |date=2006-02-01 |title=Synthesis, structure and physical properties of Ru ferrites: Ba<math>M</math>Ru₅O₁₁ (M=Li and Cu) and BaM′₂Ru₄O₁₁ (M′=Mn, Fe and Co) |url=https://www.sciencedirect.com/science/article/pii/S0022459605005396 |journal=Journal of Solid State Chemistry |language=en |volume=179 |issue=2 |pages=563–572 |doi=10.1016/j.jssc.2005.11.014 |issn=0022-4596|url-access=subscription }}</ref> The S block is essentially a slab cut along the <math>(111)</math> plane of an AB₂O₄ spinel. Each S block has one A layer and one B layer. The A layer features Me^S-centered tetrahedron and Me^S-centered octahedron, while the B layer is made up of edge-sharing Me^S-centered octahedron. Both A and B layers have the same chemical formula of Me^S₃O₄²⁺.

=== R block ===

The R block has a chemical formula of Me^LMe^S₆O₁₁^2-. Me^L are larger metal cations, for example, alkaline earth metals (Ba,<ref>{{Cite journal |last1=Nemrava |first1=Sandra |last2=Vinnik |first2=Denis A. |last3=Hu |first3=Zhiwei |last4=Valldor |first4=Martin |last5=Kuo |first5=Chang-Yang |last6=Zherebtsov |first6=Dmitry A. |last7=Gudkova |first7=Svetlana A. |last8=Chen |first8=Chien-Te |last9=Tjeng |first9=Liu Hao |last10=Niewa |first10=Rainer |date=2017-04-03 |title=Three Oxidation States of Manganese in the Barium Hexaferrite BaFe_12–xMn_xO₁₉|url=https://pubs.acs.org/doi/10.1021/acs.inorgchem.6b02688 |journal=Inorganic Chemistry |language=en |volume=56 |issue=7 |pages=3861–3866 |doi=10.1021/acs.inorgchem.6b02688 |pmid=28290672 |issn=0020-1669|url-access=subscription }}</ref> Sr,<ref name=":0">{{Cite journal |last1=Li |first1=Jun |last2=Medina |first2=Elena A. |last3=Stalick |first3=Judith K. |last4=Sleight |first4=Arthur W. |last5=Subramanian |first5=M. A. |date=2016-05-01 |title=Structural studies of CaAl₁₂O₁₉, SrAl₁₂O₁₉, La_2/3+δAl_12–δO₁₉, and CaAl₁₀NiTiO₁₉ with the hibonite structure; indications of an unusual type of ferroelectricity |journal=Zeitschrift für Naturforschung B |language=en |volume=71 |issue=5 |pages=475–484 |doi=10.1515/znb-2015-0224 |s2cid=102258634 |issn=1865-7117|doi-access=free }}</ref>) rare earth metals,<ref name=":0" /> Pb,<ref>{{Cite journal |last1=Comer |first1=J.J. |last2=Croft |first2=W.J. |last3=Kestigian |first3=M. |last4=Carter |first4=J.R. |date=March 1967 |title=An x-ray and electron microscope study of the compound PbAl₁₂O₁₉ |url=https://linkinghub.elsevier.com/retrieve/pii/002554086790013X |journal=Materials Research Bulletin |language=en |volume=2 |issue=3 |pages=293–302 |doi=10.1016/0025-5408(67)90013-X|url-access=subscription }}</ref> etc. The point group symmetry of the R block is <math>\bar{6}m2</math>. The large metal cations are located in the middle layer of the three hexagonally packed layers. This block is also composed of face-sharing Me^S-centered octahedra and Me^S-centered trigonal bipyramids.

=== T block === The T block has a chemical formula of Me^L₂Me^S₈O₁₄^2-. The point group symmetry of the T block is <math>3m</math>. One T block consists of 4 oxygen layers with the two Me^L atoms substituting two oxygen atoms in the middle two layers. In one T block, there are both Me^S-centered octahedra and Me^S-centered tetrahedra.

== Family members ==

=== M-type ferrite === M-type ferrite is made up of alternating S and R blocks in the sequence of SRS*R*. (* denotes rotating that layer around the ''c'' axis by 180°.) The chemical formula of M-type ferrite is Me^LMe^S₁₂O₁₉. Common examples are BaFe₁₂O₁₉, SrFe₁₂O₁₉. It exhibits <math>P6_3/mmc</math> space group symmetry. For BaFe₁₂O₁₉, ''a'' = 5.89 Å and ''c'' = 23.18 Å.<ref>{{Cite journal |last1=Ounnunkad |first1=S. |last2=Winotai |first2=P. |date=2006-06-01 |title=Properties of Cr-substituted M-type barium ferrites prepared by nitrate–citrate gel-autocombustion process |url=https://www.sciencedirect.com/science/article/pii/S030488530500675X |journal=Journal of Magnetism and Magnetic Materials |language=en |volume=301 |issue=2 |pages=292–300 |doi=10.1016/j.jmmm.2005.07.003 |bibcode=2006JMMM..301..292O |issn=0304-8853|url-access=subscription }}</ref> M-ferrite is a very robust ferrimagnetic material, thus widely used as fridge magnets, card strips, magnets in speakers, magnetic material in linear tape-open. thumb|M-type ferrite.

=== W-type ferrite === W-type ferrite, like the M-type, consists of S and R blocks, but the stacking order and the number of blocks are different. The stacking sequence in a W-ferrite is SSRS*S*R* and its chemical formula is Me^LMe^S₁₈O₂₇. It exhibits <math>P6_3/mmc</math> space group symmetry. One example of W-type ferrite is BaFe₁₈O₂₇, with ''a'' = 5.88 Å and ''c'' = 32.85 Å.<ref>{{Cite journal |last=Braun|first=P. B.|date=1957 |title=The crystal structures of a new group of ferromagnetic compounds |url=https://cir.nii.ac.jp/crid/1573668925777306880 |journal=Philips Res. Rep. |volume=12 |pages=491–548}}</ref>

=== R-type ferrite === R-type ferrite has a chemical formula of Me^LMe^S₆O₁₁ with a space group of <math>P6_3/mmc</math>. Unlike other hexaferrites, R-type ferrite doesn't have an S block. Instead, it only has single B layers extracted from the S block. The stacking sequence is BRB*R*.

=== Y-type ferrite === Y-type ferrite has a chemical formula of Me^L₂Me^S₁₄O₂₂ with a space group of <math>R\bar{3}m</math>. One example is Ba₂Co₂Fe₁₂O₂₂ with ''a'' = 5.86 Å and ''c'' = 43.5 Å.<ref>{{Cite journal |last1=Shin |first1=H. S. |last2=Kwon |first2=S.-J. |date=June 1993 |title=X-ray powder diffraction patterns of two Y-type hexagonal ferrites |url=http://dx.doi.org/10.1017/s0885715600017905 |journal=Powder Diffraction |volume=8 |issue=2 |pages=98–101 |doi=10.1017/s0885715600017905 |bibcode=1993PDiff...8...98S |s2cid=94413989 |issn=0885-7156|url-access=subscription }}</ref> Y-type ferrite is built up with S and T blocks with an order of 3(ST) in one unit cell. There is no horizontal mirror plane in a Y-type ferrite.

=== Z-type ferrite === Z-type ferrite has a chemical formula of Me^L₃Me^S₂₆O₄₁ with a space group of <math>P6_3/mmc</math>. It has a complicated stacking of SRSTS*R*S*T* in one unit cell. Some Z-type members may have sophisticated magnetic properties along different directions.<ref>{{Cite journal |last=Pullar |first=Robert C. |date=2012-09-01 |title=Hexagonal ferrites: A review of the synthesis, properties and applications of hexaferrite ceramics |url=https://www.sciencedirect.com/science/article/pii/S0079642512000369 |journal=Progress in Materials Science |language=en |volume=57 |issue=7 |pages=1191–1334 |doi=10.1016/j.pmatsci.2012.04.001 |issn=0079-6425|url-access=subscription }}</ref> One example is Ba₃Co₂Fe₂₄O₄₁ with ''a'' = 5.88 Å and ''c'' = 52.3 Å.

=== X-type ferrite === X-type ferrite has a chemical formula of Me^L₂Me^S₃₀O₄₆ with a space group of <math>R\bar{3}m</math>. The stacking order is 3(SRS*S*R*) in one unit cell. One example is Sr₂Co₂Fe₂₈O₄₆ with ''c'' = 83.74 Å.<ref>{{Cite journal |last=Wu |first=Meixia |title=Single-Crystal Growth and Room-Temperature Magnetocaloric Effect of X-Type Hexaferrite Sr₂Co₂Fe₂₈O₄₆ |url=https://doi.org/10.1021/acs.inorgchem.9b03724 |journal=Inorganic Chemistry|year=2020 |volume=59 |issue=10 |pages=6755–6762 |doi=10.1021/acs.inorgchem.9b03724 |pmid=32364708 |osti=1631933 |s2cid=218504683 }}</ref>

=== References === <references />

Category:Ferromagnetic materials Category:Ceramic materials