beta-tungsten

β-W was first observed by Hartmann et al. in 1931 as part of the dendritic metallic deposit formed on the cathode after electrolysis of phosphate melts below 650°C.{{Cite journal |last1=Hartmann |first1=Hellmuth |last2=Ebert |first2=Fritz |last3=Bretschneider |first3=Otto |date=1931-05-28 |title=Elektrolysen in Phosphatschmelzen. I. Die elektrolytische Gewinnung von α- und β-Wolfram |url=http://dx.doi.org/10.1002/zaac.19311980111 |journal=Zeitschrift für anorganische und allgemeine Chemie |volume=198 |issue=1 |pages=116–140 |doi=10.1002/zaac.19311980111 |issn=0863-1786|url-access=subscription }}{{Cite journal |last1=Morcom |first1=W. R. |last2=Worrell |first2=W. L. |last3=Sell |first3=H. G. |last4=Kaplan |first4=H. I. |date=January 1974 |title=The preparation and characterization of beta-tungsten, a metastable tungsten phase |url=http://dx.doi.org/10.1007/bf02642939 |journal=Metallurgical Transactions |volume=5 |issue=1 |pages=155–161 |doi=10.1007/bf02642939 |bibcode=1974MT......5..155M |issn=0026-086X|url-access=subscription }} In the beginning stages of research into β-W, oxygen was commonly found to promote the formation of the β-W structure, thus discussions of whether the β-W structure is a phase of single-element tungsten or a tungsten suboxide were long-standing,{{Cite journal |last1=Hägg |first1=G. |last2=Schönberg |first2=N. |date=1954-04-01 |title='β-Tungsten' as a tungsten oxide |journal=Acta Crystallographica |volume=7 |issue=4 |pages=351–352 |doi=10.1107/s0365110x54000989 |issn=0365-110X|doi-access=free |bibcode=1954AcCry...7..351H }} but ever since the 1950s there has been a lot of experimental proof showing that the oxygen in β-W thin films is in a zero valence state,{{Cite journal |last1=Petroff |first1=P. |last2=Sheng |first2=T. T. |last3=Sinha |first3=A. K. |last4=Rozgonyi |first4=G. A. |last5=Alexander |first5=F. B. |date=June 1973 |title=Microstructure, growth, resistivity, and stresses in thin tungsten films deposited by rf sputtering |url=http://dx.doi.org/10.1063/1.1662611 |journal=Journal of Applied Physics |volume=44 |issue=6 |pages=2545–2554 |doi=10.1063/1.1662611 |bibcode=1973JAP....44.2545P |issn=0021-8979|url-access=subscription }}{{Cite journal |last1=Mannella |first1=G. |last2=Hougen |first2=J. O. |date=August 1956 |title="β-Tungsten" as a Product of Oxide Reduction |url=http://dx.doi.org/10.1021/j150542a035 |journal=The Journal of Physical Chemistry |volume=60 |issue=8 |pages=1148–1149 |doi=10.1021/j150542a035 |issn=0022-3654|url-access=subscription }}{{Cite journal |last1=Shen |first1=Y.G |last2=Mai |first2=Y.W |date=May 2000 |title=Influences of oxygen on the formation and stability of A15 β-W thin films |url=http://dx.doi.org/10.1016/s0921-5093(00)00745-0 |journal=Materials Science and Engineering: A |volume=284 |issue=1–2 |pages=176–183 |doi=10.1016/s0921-5093(00)00745-0 |issn=0921-5093|url-access=subscription }} and thus the structure is a true allotrope of tungsten.

While the initial interest in β-W thin films was driven by its superconducting properties at low temperatures,{{Cite journal |last1=Basavaiah |first1=S. |last2=Pollack |first2=S. R. |date=1968-04-15 |title=Superconductivity in Evaporated Tungsten Films |journal=Applied Physics Letters |volume=12 |issue=8 |pages=259–260 |doi=10.1063/1.1651982 |issn=0003-6951|doi-access=free |bibcode=1968ApPhL..12..259B }} the discovery of giant spin Hall effect in β-W thin films by Burhman et al. in 2012 has generated new interest in the material for potential applications in spintronic magnetic random access memories and spin-logic devices.

β-W has a cubic A15 structure with space group $Pm\backslash bar\{3\}n$, which belongs to the Frank–Kasper phases family. Each unit cell contains eight tungsten atoms. The structure can be seen as a cubic lattice with one atom at each corner, one atom in the center, and two atoms on each face. There are two inequivalent tungsten sites corresponding to Wyckoff positions $2a$ and $6c$, respectively. On the first site, Wyckoff position $2a$, each tungsten atom is bonded to twelve equivalent W atoms to form a mixture of edge- and face-sharing WW₁₂ cuboctahedratungsten. On the second site, with Wyckoff position $6c$, each tungsten atom is bonded to fourteen neighboring tungsten atoms, and there is a spread of W–W bond lengths ranging from 2.54 to 3.12 Å.{{Cite web |title=mp-11334: W (Cubic, Pm-3n, 223) |url=https://materialsproject.org/materials/mp-11334?chemsys=W#literature_references |access-date=2022-12-13 |website=Materials Project}} The experimentally measured lattice parameter of β-W is 5.036 Å, while the DFT calculated value is 5.09 Å.

Two key properties of β-W have been well-established: the high electrical resistivity and the giant spin Hall effect.

Although the exact value depends on the preparation conditions, β-W has an electrical resistivity of at least five to ten times higher than that of α-W (5.3 μΩ.cm),{{Cite journal |last1=Petroff |first1=P.M. |last2=Reed |first2=W.A. |date=March 1974 |title=Resistivity behavior and phase transformations in β-W thin films |url=http://dx.doi.org/10.1016/0040-6090(74)90092-3 |journal=Thin Solid Films |volume=21 |issue=1 |pages=73–81 |doi=10.1016/0040-6090(74)90092-3 |bibcode=1974TSF....21...73P |issn=0040-6090|url-access=subscription }}{{Cite journal |last1=Desai |first1=P. D. |last2=Chu |first2=T. K. |last3=James |first3=H. M. |last4=Ho |first4=C. Y. |date=October 1984 |title=Electrical Resistivity of Selected Elements |url=http://dx.doi.org/10.1063/1.555723 |journal=Journal of Physical and Chemical Reference Data |volume=13 |issue=4 |pages=1069–1096 |doi=10.1063/1.555723 |bibcode=1984JPCRD..13.1069D |issn=0047-2689|url-access=subscription }}{{Cite journal |last1=Barmak |first1=Katayun |last2=Liu |first2=Jiaxing |last3=Harlan |first3=Liam |last4=Xiao |first4=Penghao |last5=Duncan |first5=Juliana |last6=Henkelman |first6=Graeme |date=2017-10-21 |title=Transformation of topologically close-packed β-W to body-centered cubic α-W: Comparison of experiments and computations |url=http://dx.doi.org/10.1063/1.4995261 |journal=The Journal of Chemical Physics |volume=147 |issue=15 |pages=152709 |doi=10.1063/1.4995261 |pmid=29055319 |bibcode=2017JChPh.147o2709B |issn=0021-9606|url-access=subscription }} and this high conductivity{{Contradictory inline|date=January 2025|reason=Previous text states that β-W has a higher resistivity than α-W}} will remain almost unchanged in a temperature range of 5 to 380 K,{{Cite journal |last1=Hao |first1=Qiang |last2=Chen |first2=Wenzhe |last3=Xiao |first3=Gang |date=2015-05-04 |title=Beta (β) tungsten thin films: Structure, electron transport, and giant spin Hall effect |url=http://dx.doi.org/10.1063/1.4919867 |journal=Applied Physics Letters |volume=106 |issue=18 |pages=182403 |doi=10.1063/1.4919867 |bibcode=2015ApPhL.106r2403H |issn=0003-6951|url-access=subscription }} making β-W a potential thin film resistor while α-W is a thin film conductor.

Thin films of β-W display a giant spin Hall effect with a spin Hall angle of 0.30 ± 0.02 and a spin-diffusion length of around 3.5 nm. In contrast, α-W exhibits a much smaller spin Hall angle of less than 0.07 and a comparable spin-diffusion length. In the spin Hall effect, the application of a longitudinal electric current through a nonmagnetic material generates a transverse spin current due to the spin–orbit interaction, and the spin Hall angle is defined as the ratio of the transverse spin current density and the longitudinal electric current density. The spin Hall angle of β-W is large enough to generate spin torques capable of flipping or setting the magnetization of adjacent magnetic layers into precession by means of the spin Hall effect.

While there have been some reports about preparing β-W with chemical methods such as hydrogen reduction reaction,{{Cite journal |last1=FEDERER |first1=J. I. |last2=STEELE |first2=R. M. |date=February 1965 |title=Identification of a Beta-Tungsten Phase in Tungsten–Rhenium Alloys |url=http://dx.doi.org/10.1038/205587b0 |journal=Nature |volume=205 |issue=4971 |pages=587–588 |doi=10.1038/205587b0 |bibcode=1965Natur.205..587F |issn=0028-0836|url-access=subscription }} almost all the reported β-W in the recent thirty years are prepared through sputter deposition, an atom-by-atom physical vapor deposition (PVD) technique.{{Cite journal |last1=Chattaraj |first1=Ananya |last2=Asirvatham |first2=Joshua |last3=Das |first3=Gangadhar |last4=Manna |first4=Gouranga |last5=Saha |first5=Pinku |last6=Kumar |first6=Vijay |last7=Kanjilal |first7=Aloke |date=2022-03-28 |title=Growth-dependent structural ordering and stability in β-tungsten films for spintronic applications |url=http://dx.doi.org/10.1063/5.0087436 |journal=Journal of Applied Physics |volume=131 |issue=12 |pages=125301 |doi=10.1063/5.0087436 |bibcode=2022JAP...131l5301C |issn=0021-8979|url-access=subscription }}{{Cite journal |last1=O’Keefe |first1=M. J. |last2=Grant |first2=J. T. |last3=Solomon |first3=J. S. |date=August 1995 |title=Magnetron sputter deposition of A-15 and bcc crystal structure tungsten thin films |url=http://dx.doi.org/10.1007/bf02652968 |journal=Journal of Electronic Materials |volume=24 |issue=8 |pages=961–967 |doi=10.1007/bf02652968 |bibcode=1995JEMat..24..961O |issn=0361-5235|url-access=subscription }} In the sputter deposition, a tungsten target is bombarded with ionized gas molecules (usually Ar), causing the tungsten atoms to be “sputtered” off into the plasma. These vaporized atoms are then deposited when they condense as a thin film on the substrate to be coated. The formation of β-W through sputter deposition depends on the base pressure, Ar pressure, substrate temperature, impurity gas, deposition rate, film thickness, substrate type, etc.{{Cite journal |last1=Barmak |first1=Katayun |last2=Liu |first2=Jiaxing |date=2017-11-01 |title=Impact of deposition rate, underlayers, and substrates on β-tungsten formation in sputter deposited films |url=https://avs.scitation.org/doi/full/10.1116/1.5003628 |journal=Journal of Vacuum Science & Technology A |volume=35 |issue=6 |pages=061516 |doi=10.1116/1.5003628 |issn=0734-2101|doi-access= |bibcode=2017JVSTA..35f1516B |url-access=subscription }}{{Cite journal |last1=Vink |first1=T. J. |last2=Walrave |first2=W. |last3=Daams |first3=J. L. C. |last4=Dirks |first4=A. G. |last5=Somers |first5=M. A. J. |last6=van den Aker |first6=K. J. A. |date=1993-07-15 |title=Stress, strain, and microstructure in thin tungsten films deposited by dc magnetron sputtering |url=http://dx.doi.org/10.1063/1.354842 |journal=Journal of Applied Physics |volume=74 |issue=2 |pages=988–995 |doi=10.1063/1.354842 |bibcode=1993JAP....74..988V |issn=0021-8979|url-access=subscription }}{{Cite journal |last1=Weerasekera |first1=I. A. |last2=Shah |first2=S. Ismat |last3=Baxter |first3=David V. |last4=Unruh |first4=K. M. |date=1994-06-13 |title=Structure and stability of sputter deposited beta-tungsten thin films |url=http://dx.doi.org/10.1063/1.111318 |journal=Applied Physics Letters |volume=64 |issue=24 |pages=3231–3233 |doi=10.1063/1.111318 |bibcode=1994ApPhL..64.3231W |issn=0003-6951|url-access=subscription }} It has been widely observed that oxygen or nitrogen gas flow can assist and is necessary for the formation of β-W, but recently there have also been reports on preparing β-W without putting into any impurity gas during deposition.{{Cite journal |last1=Nagakubo |first1=A. |last2=Lee |first2=H. T. |last3=Ogi |first3=H. |last4=Moriyama |first4=T. |last5=Ono |first5=T. |date=2020-01-13 |title=Elastic constants of beta tungsten thin films studied by picosecond ultrasonics and density functional theory |url=https://aip.scitation.org/doi/full/10.1063/1.5131768 |journal=Applied Physics Letters |volume=116 |issue=2 |pages=021901 |doi=10.1063/1.5131768 |bibcode=2020ApPhL.116b1901N |issn=0003-6951|hdl=11094/83934 |hdl-access=free }}{{Cite journal |last1=Zhu |first1=Fei |last2=Xie |first2=Zheng |last3=Zhang |first3=Zhengjun |date=2018-03-01 |title=Phase control and Young's modulus of tungsten thin film prepared by dual ion beam sputtering deposition |journal=AIP Advances |volume=8 |issue=3 |pages=035321 |doi=10.1063/1.5021009|doi-access=free |bibcode=2018AIPA....8c5321Z }}

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Category:Tungsten

Category:Allotropes