OIE 31)
2. Rare earth arsenates
2.1. Introduction
There are many common features to the structural chemistry of rare earth compounds formed by the tetrahedral oxoanions RXO4, where X is a pentavalent element (P, As, V, Cr). Nevertheless, considerably less structural and other information is available for the rare earth arsenates than for the phosphates and vanadates.
2.2. Binary a r s e n a t e s
Rare earth arsenates can be prepared by firing stoichiometric amounts of rare earth nitrate and ammonium arsenate for several hours at 700-1150°C (Schwarz, 19630; or they can be precipitated in water solution by NazHAsO 4 solution. The product in the case of the lighter lanthanides is anhydrous arsenate, in contrast to
INORGANIC COMPLEX COMPOUNDS II 141 the corresponding phosphate, which is a hydrate. RAsO 4 differs from RVO 4 in that the structure is independent of the method of preparation (Escobar and Baran, 1978). The heavier lanthanides, yttrium and scandium, form the hydrated compound ScAsO 4 . 2 H 2 0 in the reaction between ScC13 and Na2HAsO4, or Sc20 3 and H3AsO 4, at 25°C (Ivanov-Emin et al., 1971; Shakhtakhtinskii et al., 1977).
Optically clear RAsO 4 crystals were grown from Pb2As20 7 flux by slow cooling and recovered by a hot-pouring technique (Smith et al., 1978). Attempts to prepare larger crystals by modifying the flux and the controlling of evaporation by a crystallization seal have been made by Wanklyn et al. (1984). Both monoclinic and tetragonal orthoarsenates were grown.
The structural similarity between YPO 4 and YAsO 4 was observed over fifty years ago by Strada and Schwendimann (1934). Most of the rare earth arsenates ( S m . . . L u ) have been found to have the tetragonal zircon structure (Durif, 1956), whereas the larger rare earths, like the corresponding phosphates, have the monoclinic monazite structure (Carron et al., 1958). The behavior of the neigh- boring arsenates NdAsO 4 and SmAsO 4 suggests and PmAsO 4 will be dimorphic, but this compound has not been prepared (Escobar and Baran, 1978). At higher pressures (2000-7000 MPa), the arsenates have scheelite structure. The phase transition zircon---~ scheelite has been observed for most arsenates (Sm. • • Lu, Y).
With PrAsO 4 and NdAsO 4 the monazite--~scheelite transformation is possible (Stubican and Roy, 1962). The vibrational spectra of RAsO 4 recorded by Botto and Baran (1982) are closely similar to those of other R X O 4 compounds (fig. 41).
In table 5 a summary of the structural data for rare earth arsenates is given.
The formation of different scandium arsenates in solution has been studied by titration (Chernova et al., 1974). The formation of compounds with Sc 3+ :AsO34 - ratios of 1 : 1, 1 : 1.5, and 1 : 3 was confirmed.
Khrameeva et al. (1971) have prepared both anhydrous scandium arsenate, Sc(AsO3)3, and hydrogenarsenate dihydrate, Sc(HAsOe)3"2H20. The former compound is very soluble in water, and the latter hydrolyzes to ScAsOg.2H20.
The Sc(AsO3) 3 compound decomposes upon heating to ScAsO 4 at 800-900°C.
The decomposition of Sc(H2AsO4) 3 . 2 H 2 0 is a multistep process and the fol- lowing intermediate products have been obtained: Sc(H2AsO4) 3 • H 2 0 , Sc(H2AsO4)3, Sc2(H2As207)3, Sc(AsO3)3, and S c A s O 4 (fig. 42).
The same group (Khrameeva et al., 1973) has also used IR and NMR spectroscopy to study Sc2(HAsO4) 3 • n H 2 0 (n = 0, 1, 2) and Sc4(As207) 3. Start- ing material in this study was Sc2(HAsO4) 3 . 2 H 2 0 , which yielded the other compounds upon heating in air. Sc4(As207)3 was found to be the most stable of the compounds; its thermal stability range extended from 330 to 650°C.
Scandium arsenate dihydrate ScAsO4.2H20 is isomorphic with the corres- ponding phosphate, differing from it only in the somewhat higher solubility in water and reduced thermal stability (Komissarova et al., 1971c).
142
(a)
(b)
~2
g
L. NIINISTO and M. L E S K E L J k
i i i
I I I I I I I
900 760 [ 5 6 0 1 3 6 0
[cm-,]
Fig. 41. The vibrational spectra of L a A s O 4.
(a) R a m a n spectrum, (b) IR spectrum.
( B o t t o and Baran, 1982.)
TABLE 5
Summary of structural data for rare earth arsenates.
C o m p o u n d R Example a b c /3 Z Space Ref.
(,~) (/~) (,~) (deg) group
R A s O 4 L a . • • Nd Ce 7.02 7.15 6.59 104.3 4 P21 a
S m . • • Lu, Y, Sc Eu 7.17 6.35 4 I41/amd b
R A s O 4 - 2H20 Sc Sc 5.64 10.47 9.36 c
(a) Botto and Baran (1982).
(b) Durif and Forrat (1957).
(c) Komissarova et al. (197!c).
I N O R G A N I C C O M P L E X C O M P O U N D S II 143 t, ° C
/000
800
200 0
~ 2 0 4n
e-
~ D T ~
- - - <
V'00
TG
Fig. 42. The TG, D T G , and D T A cur- ves for Sc(H2AsO4) 3 -2H20. Heating rate 10°C/min (Khrameeva et al., 1971).
2.3. Double and triple a r s e n a t e s
Like the corresponding phosphates, NaBR(AsO4) 2 compounds have two low- temperature forms: orthorhombic NaBNd(PO4) z type and monoclinic Na3Nd- (VO4) 2 type (fig. 43). The region of the first type in the rare earth series is narrower in arsenates ( L a - . . Nd) than in phosphates ( L a - - . Eu) (Vlasse et al.,
" < - - - -
© ~ ® 1
900 "~
®
Xj®
'X
700 @ . . .
5OO
t
' ' ' G'd ' D'I . . . .
Nd Sm Eu Tb Ho Er Tm Yb Lu
Fig. 43. The thermal behavior of the Na3R(AsO4) 2 phases. Abbreviations have been presented in fig.
31 (Vlasse et al., 1980b).
144 L. NIINISTO and M. LESKELA
1980b). The four higher-temperature forms of sodium rare earth arsenates are likewise similar to those of phosphates. Na3R(AsO4) 2 (R = L a . . . N d ) have a glaserite-type high-temperature form. The rare earths from Sm to Tb have an orthorhombic intermediate phase identical in structure with orthorhombic Na 3- Nd(PO4)2, and a high-temperature phase of glaserite-type. The medium-tempera- ture phase of arsenates from Dy to Lu is the same as observed for Na3Tm(PO4) 2.
The high-temperature phase of Na3Dy(AsO4) 2 and Na3Ho(AsO4) 2 is hexagonal glaserite-type, and compounds of E r . . . Lu have hexagonal Na3Yb(PO4) 2 struc- ture (Parent et al., 1980b).
KzCO 3 (or Rb2CO3), H3AsO4, and R A s O 4 react in solid state at 500°C to give K3R(AsO4) 2 (Kalinin et al., 1978). Most of the trigonal structures obtained can be considered as derived from the mineral glaserite (Melnikov and Komissarova, 1981).
Two sodium scandium arsenates, NaBSC2(AsO4) 3 and Na2HSc2(AsO4) 3
• 1.5H20, have been synthesized by Ivanov-Emin et al. (1971). Their IR spectra and X-ray diffraction patterns have been recorded but the structures are un- known.
Ternary rare earth arsenates having hexagonal apatite structure have been prepared by Escobar and Baran (1982a). As with phosphates and vanadates their composition can vary widely.
Recently, Nabar and Sakhardande (1985a,b) have studied the triple orthoarse- nates with general formula MRTh(AsO4) 3 where M = Ca, Cd. The X-ray diffrac- tion and IR spectroscopic studies have revealed that, like the binary orthoarse- nates, they have two structure types: monoclinic monazite ( L a . . . N d ) and tetragonal zircon ( S i n - - - T m , Y). Some of the compounds show dimorphism at high temperatures (>900°C): a monazite--+ scheelite transition.
2.4. Properties of rare earth arsenates 2.4.1. Chemical properties
Because the rare earth arsenates are very insoluble, Na3AsO 4 can be used in the quantitative determination of rare earth ions (Shakhtakhtinskaya and Isken- derov, 1973, 1975; Shakhtakhtinskii et al., 1977)•
Ce(IV) forms hydrogenarsenate dihydrate when precipitated with NazHAsO 4.
Its dehydration, intercalation reactions, ion exchange properties, and possible use in radiochemical applications have extensively been studied by Zsinka et al.
(Zsinka et al., 1974; Zsinka and Szirtes, 1974; Szirtes and Zsinka, 1974; Korney et al., 1977, 1978a,b; Szirtes et al., 1984)•
The thermal stability of rare earth arsenate dihydrates, the formation of R A s O 4 phases of various structures, and the melting of R A s O 4 have been studied by Angapova and Serebrennikov (1973a,b). The melting points of R A s O 4 are between 1830 and 2000°C, increasing with increasing atomic number•
I N O R G A N I C C O M P L E X C O M P O U N D S II 145
2.4.2.
Physical properties
While rare earth phosphates and vanadates are extensively investigated, the literature on the properties of analogous arsenates is rather sparse. T b A s O 4 , DyAsO4, and TmAsO 4 are most interesting because of their structural and magnetic transitions at low temperatures.
Due to the Jahn-Teller effect TbAsO 4 transforms to a structure of low symmetry at 27.7K and DyAsO 4 at 11.2K (Klein et al., 1971; Hudson and Magnum, 1971; Will et al., 1971b; Goebel and Will, 1972a,b; Wappler, 1974).
Both structures have been studied by X-ray and neutron diffraction techniques.
After the transition the unit cell of DyAsO 4 is a = 7.06 A, b = 7.03 A, c = 6.30 A (Imma) and that of TbAsO 4 a = 10.12A, b = 9.95 A, c = 6.31 A (Fddd) (Long
and Stager, 1977). __
group of P212121 (Will et al., 1971a; Schaefer and Will, 1971; Goebel and Will, 1972c). In T b A s O 4 the corresponding magnetic transition from paramagnetic to the antiferromagnetic state occurs at 1.50K (Becher et al., 1972; Klein, 1973).
Below the transition temperature, TbAsO 4 is an Ising ferromagnet with induced magnetic moments (Mueller et al., 1983). TmAsO 4 undergoes a magnetically controllable Jahn-Teller distortion at 6 K (Magnum et al., 1971; Battison et al., 1976). This transition and the connected electronic properties of TmAsO 4 have also been studied by M6ssbauer spectroscopy (Hodges et al., 1982), NMR spectroscopy, and high resolution optical absorption spectroscopy in the magnetic field (Bleaney et al., 1983, 1984).
The dielectric measurements of R A s O 4 crystals have shown them to be ferroelectric, with Curie temperatures in the region of 25-55°C (fig. 44) (Ismail- zade et al., 1980).
2.4.3.
Spectroscopical properties
The luminescence properties of E u 3+ i n R A s O 4 have been reported already in 1966 but the arsenates have not found use in lamps as luminescent materials (Wanmaker et al., 1966). The IR excited visible luminescence has been obtained with Yb 3+ and Er 3+ in arsenates but the conversion efficiency is much lower than that obtained in some fluorides (Sommerdijk et al., 1971). The rare earth YAsO 4 host lattice interactions have been reviewed and the crystal field parameters for various R 3+ ions in this site symmetry have been presented (Wortman et al., 1976). Vishwamittar (1974a,b) has studied the crystal field parameters of Er 3+ in zircon-type arsenate structures. The multiphonon relaxation rates of excited states of R 3+ ions in yttrium arsenate have been investigated by Reed and Moos (1973a,b).
Vibrational spectroscopies have been employed in characterization of different structure types of R A s O 4. Raman scattering investigations have been made at low
146 L. NIINISTC) and M. LESKEL,~
400 ,~f~ o Pr As 04
* E u A s O 4
E • Gd As 04
• T b A $ 0 4
300 h ~ k ~ \ *
7Xi
50 100 150
TC°C)
Fig. 44. The dielectric constant versus temperature for the RAsO 4 compounds (after Ismail- zade et al., 1980).
temperatures to indicate the phase transitions occurring in Tb, Dy, and Tm arsenates (D'Ambrogio et al., 1971; Harley et al., 1971, 1972; Elliot et al., 1972).
EPR measurements of several R 3+ ions (e.g., Gd 3+, Dy 3+, Tb 3+, Er 3+, Tm 3+, Yb 3+) in tetragonal RAsO 4 (R is usually Y) have been carried out. The g-factors, hyperfine constants, magnetic interaction, symmetry parameters, and pair interac- tions of R 3+ ions were determined in these studies (Schowalter, 1971; Kalbfleisch, 1972; Hillmer et al., 1972; Schwab, 1975; Schwab and Hillmer, 1975; Mehran et al., 1979).