Binary Lanthanide Oxides

126 Pages·1997·2.26 MB·English

by  W. Suski

Ref. p. 121] 2.1.1 Introduction 1
2 Magnetic properties of binary lanthanide and actinide oxides
2.1 Binary lanthanide oxides
2.1.1 Introduction
This chapter presents a selection of data concerning the magnetic and related properties of the binary
lanthanide oxides and some related compounds obtained since 1980, and can be regarded as supplement
to Chapter 6.2 of Vol. III/12c, compiled by U. Köbler and Ch. Sauer [LB2].
A comparison of the previously published Survey [LB2] with the present one reveals several
differences. The majority of the magnetic data obtained either from magnetometric measurements or from
neutron diffraction experiments were published before 1980. Therefore, for the present data the main
emphasis is devoted to "related" properties without which, however, the understanding of classical
magnetic properties is impossible. As an example of such related properties the M -XA spectra of the
IV-V
R O -type oxides are demonstrated in Fig. 1i, a and b [84KKBP].
2 3
The other important difference between LB2 and the present data is the number of data obtained on
single crystal samples since 1980 (see e.g. [80GMDA, 80MEGD, 85GASS, 86WSK, 87Sc, 87TP,
88COLL, 88SVGA, 90AT, 90SGCK]) and also on thin films (see e.g. [81SAVG, 85Al, 85BB, 89JV]). A
review of this last subject is also presented in [89Ga], whereas a review of the synthesis methods is given
in [91MM] . The more well-defined materials allow to reexamine the R-O phase diagrams and as an
example the phase diagram of TbO is shown in Fig. 2i [93ZKE]. Some new facts about the CeO
x x
diagram for x > 1.5 obtained by heat capacity and thermal expansion coefficient measurements are
presented [89KRNR].
The intermediate (IV) or mixed valence (MV) state is the phenomenon which has been discovered in
the late sixties but still attracts some attention. The mixed or intermediate valence state in the present
review is considered as being the coexistence of two valence species at the same crystallographic site
with different life-times. Among the reported oxide compounds, SmO and EuO are supposed to exhibit
the IV state contrary to CeO where it seems that the IV state is finally excluded (see e.g. [82Wa]). The
2
existence of the IV state is also possible for other compounds. In Table 1i we indicate the ions which are
potential candidates to exhibit the IV state. For SmO magnetic and transport experiments supply an
indication of IV behavior [80KRKA]. EuO is broadly investigated by many possible physical and
chemical methods. It seems that EuO exhibits a valence u = 2.5 under a pressure of 35 GPa [84ZTSF],
but the interest in this material results not only from its IV properties but also since EuO is considered as
a model Heisenberg ferromagnet and therefore a number of sophisticated experimental as well as
theoretical investigations are still being carried out. For both these reasons and also due to its
semiconducting properties in the normal state, this material is examined not only in pure form but also in
the nonstoichiometric or doped modification (for a review see [86MG]). The nonstoichiometric forms of
other oxides were obtained and investigated too (e.g. Er2O3 [87YPCP] or CeO2- x[88CB]).
The various valence forms of the lanthanides correspond to various stoichiometry of the oxides.
According to [88El] the lanthanide oxides can be classified into three capital groups:
1. Lower oxides (monoxides, and R O -type oxides where R = Eu and Yb).
3 4
2. Sesquioxides.
3. Higher oxides with O/R ‡ 1.5.
The majority of monoxides can be obtained by high-temperature and high-pressure processes
(see[LB2] and [81LYL]) and their stability range is not yet well established. The valence of the
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New Series III/27C1
2 2.1.1 Introduction [Ref. p. 121
lanthanide atom in these compounds could be an intriguing question. This problem for SmO and EuO
was mentioned above but CeO and YbO can be also considered as the candidates for the intermediate
valence.
Since information on the R O -type oxides is rather scarce (R=Eu,Yb), they are not represented in the
3 4
main Table (Survey, sect. 2.1.3), however some attention will be devoted to them here. They exhibit the
orthorhombic crystal structure of the CaFe O -type [83AKB]. In Eu O and Yb O there should be two
2 4 3 4 3 4
stable valence species of two nonequivalent crystallographic positions. The coordination polyhedra of
oxygen ions around the R3+ ions is 6-fold in the form of a distorted octahedron; around the R2+ ions the
coordination is 8-fold in the form of a trigonal prism with two additional oxygen ions capping two faces
of the prism [83AKB]. Therefore, these compounds are the examples of stable mixed valence (see e.g.
[67Gl]) in a contrast to the fluctuating valence observed for the monoxides.
The stoichiometry of the sesquioxides favorizes the 3+ ion of lanthanide. However, their phase
relations are complicated due to the existence of 5 crystallographic modifications. Under particular
conditions some of these modifications can coexist and an isolation of pure single phase species can
create an unsolvable problem. We remind that the crystallographic forms are as follows: a) cubic (bcc) C
modification, b) hexagonal A modification and c) monoclinic B modification, and two high temperature
modifications, d) hexagonal H and e) cubic X modifications. The existence range of polymorphic forms
and transition temperatures were shown in [LB2, p.309, Fig. 412]. The crystal structures of the lanthanide
sesquioxides, R O , have been systematized according to the radius ratio, h , between the lanthanide ion
2 3
crystal radius and the "equivalent formula unit radius" which is derived from the unit cell volume of the
sesquioxide [92CP]. These systematics are shown in Fig. 3i. It follows that phase transformations of the
lanthanide sesquioxides occur for the following critical values of h : B fi A at h = 2.26 and C fi B at h =
2.42.
In the composition range R O -RO (R = Ce, Pr and Tb) there are the compounds with the defect
2 3 2
CaF2 crystal structure represented by the formula RnO2n- 2k with n = 7, 9, 10, 11, 19, 24, 31, 40, 48 and k
= 1, 2, 3... [91SBE, 92Sc].
Because of the relationship to the fluorite parent structure, superlattices are formed and twinning is
one of the problems which cannot be avoided during a topotactical reaction. Therefore, X-ray single
crystal analysis is limited in use and almost all the evidence for the structures comes from domains
investigated by electron diffraction and HREM [92Sc]. According to this analysis the discussed oxides
exhibit exactly determined narrow stability ranges. Distortion of the fluorite structure follows the
particular thermal conditions of synthesis and oxygen concentration. The observed phase transitions
between the phases are frequently classified into reconstructive transitions and displacive transitions
[92Sc]. Fig. 47 in the Survey presents the relationship for the known intermediate Pr oxides as an
illustration [91SBE].
For the RO -type compounds the perfect fluorite type structure is observed. This stoichiometry can
2
suggest the 4+ ion of lanthanide to be present and as mentioned above this valence state is confirmed for
CeO [82Wa].
2
The structural data of the lanthanide oxides are collected in Table 1i [88El,92Sc].
In the Survey also results of new experimental methods are reported as e.g. HREM [89SE, 91SBE]
for structural investigations or perturbed angular correlations (see e.g.[92SWWU]).
Finally, we would like to stress that in this supplement a new organization of data is adopted. Instead
of the previously used distinctly separated Tables containing all compounds with the same structure, with
physical and chemical data ordered chronologically followed by figures, now the compact description of
each compound is used. The compounds are presented according to their lanthanide element and
according to increasing oxygen contents (e.g. RO, R O , RO ).
2 3 2
For each compound the following properties are listed:
I. Electronic structure including results of X-ray spectroscopy (XPS, XAS, XANES, BIS, etc) and
de Haas-van Alphen experiments as well as the results of theoretical considerations.
II. Crystal structure including structure type, space group, lattice parameters, with reference to the
influence of temperature and pressure.
III. Magnetic properties: magnetometric measurements.
Landolt-Börnstein
New Series III/27C1
Ref. p. 121] 2.1.1 Introduction 3
IV. Neutron diffraction: magnetic structures, elastic scattering, polarized neutrons, inelastic scattering
(INS), crystal field (CEF), lattice effects etc.
V. Nuclear methods: Mössbauer effect, NMR, EPR and m SR.
VI. Transport properties: electrical and thermal conductivity, Hall effects, magnetogalvanic effects,
thermoelectric power, magnetoresistivity.
VII. Thermodynamic properties: heat capacity, entropy, Schottky anomalies, magnetocaloric effects.
VIII. Others: elastic constants, magnetostriction, acoustic effects, optical properties, etc.
This collection of structural, magnetic and related data for individual elements is followed by the
figures and special tables (section 2.1.4) whereas the references of all compounds are located at the end
of the chapter (section 2.1.5).
Table 1i. Survey of crystallographic data for the lanthanide oxides [88El, 92Sc].
Oxide type Symmetry Space group Structure R Ions with IV
type state
RO cubic Fm3m NaCl La-Nd, Sm-Gd, Ce, Sm, Eu,
Tb, Er Yb(?)
R O orth. Pnma CaFe O Eu, Yb Eu, Yb
3 4 2 4
R O cubic C-type Ia3 Tl O La*-Lu Ce
2 3 2 3
hex. A-type P32/m La-Pm Ce(?)
monocl. B-type C121 Pm*-Lu
m
HT-hex. H-type P6 /mmc La, Ce, Nd,
3
Sm-Tm
HT-cubic Im3m La, Pr, Nd, Eu
X-type
R O rhomb. R3 dist. CaF Ce, Pr, Tb Ce, Pr, Tb
7 12 2
R O triclinic P1 dist. CaF Ce,Pr Ce,Pr
9 16 2
R O triclinic P1 dist. CaF Pr
10 18 2
R O triclinic P1 dist. CaF Tb
31 56 2
R O rhomb. P1(?) dist. CaF Ce,Pr,Tb Ce,Pr,Tb
11 20 2
R O dist. CaF Ce
19 34 2
R O monocl. P1(?) dist. CaF Pr,Tb
24 44 2
RO cubic bcc dist. CaF Pr
1.63 2
RO cubic fcc dist. CaF Pr
1.820 2
RO cubic Fm3m CaF Ce,Pr,Tb
2 2
* Note the difference with Fig. 3i.
Landolt-Börnstein
New Series III/27C1
4 2.1.1 Introduction [Ref. p. 121
Fig. 1i. R O . The M -XA spectra of the rare earth separated line groups at the 3d (M ) and 3d (M )
2 3 IV-V 5/2 V 3/2 IV
sesquioxides. (a) R = La, Nd, Sm, Eu and Gd and positions. The fine structure of these patterns is
CeO ; (b) R = Dy, Ho, Er, Tm and Yb, and Tb O governed by the 3d94fn+1 exchange splitting, thus
2 11 20
[84KKBP]. Total electron yield is proportional to the containing information on the 4f occupancy (valence)
XA coefficient. The spectra consist of two well of the initial state.
Landolt-Börnstein
New Series III/27C1
Ref. p. 121] 2.1.1 Introduction 5
Fig. 3i. R O . The radius ratio parameter, h = R /R3+
2 3 m
(R : equivalent formula unit radius, for definition see
m
original paper) versus number of the 4f electrons for
three various types of structure (A: hexagonal, B:
monoclinic and C: cubic) [92CP]. It becomes clearly
evident that the A-type crystal structure is stable when
Fig. 2i. TbO . Phase diagram showing the higher h < 2.25, the B-type structure is stable when 2.30 < h £
oxides in the Txb O -TbO system. Phase signed as f 2.42, and the C-type structure is stable when h ‡ 2.43.
2 3 2 Within each structure type, h increases with the number
means typical for R O , cubic C-type, phase s typical
2 3 of 4f electrons, consistent with the well-known
for the range RO ...RO has cubic symmetry with
1.50 1.70 lanthanide contraction.
Ia3 space group, phase a characteristic for the range
RO ...RO has cubic CaF structure (space group
1.72 2.00 2
Fm3m). In the low temperature region the following
phases exist: i: (Tb O ), rhombohedral structure, d ':
7 12
(probably Tb O or Tb O ), triclinic, d : (Tb O ),
62 112 31 56 11 20
rhombohedral (?) and b : (Tb O or Tb O ), triclinic
24 44 44 88
or monoclinic (?). For more details see [88E1, 92Sc,
93ZKE].
Landolt-Börnstein
New Series III/27C1
6 2.1.2 List of symbols and abbreviations [Ref. p. 121
2.1.2 List of frequently used symbols and abbreviations
Symbols
Symbol Unit Property
a, b, c nm lattice parameters
B T, G magnetic induction
B hyperfine magnetic field
hf
B transferred hyperfine field of nearest,next nearest
1,2
neighbors
B Pa, bar bulk modulus (B ’= dB /dp)
0 0 0
c J g- 1 K- 1 specific heat capacity
C J mol- 1 K- 1 heat capacity
C lattice contribution to heat capacity
L
C magnetic heat capacity
magn
C heat capacity at constant pressure
p
C F electrical capacity
d m m thickness, diameter
Dq eV crystal field splitting parameter
E eV, Ry, cal energy
E activation energy (for conductivity, mobility, ...)
a
E binding energy
b
E Fermi energy
F
E energy gap
g
E relative energy
r
D E mm s- 1 electric quadrupole splitting
Q
g spectroscopic splitting factor
H Oe magnetic field (strength), mostly given as m H in
0
tesla (T)
H coercive force, critical field
c
H magnetic hyperfine field
hf
DH Oe linewidth of resonance spectrum
D H Oe spin wave linewidth
k
I intensity [various units]
IS mm s- 1 isomer shift
J eV exchange interaction energy (J/k in K)
B
J exchange interaction energies for nearest and next
1,2
nearest neighbors
J total orbital angular momentum quantum number
k nm- 1, cm- 1 wavevector
k J K- 1 Boltzmann constant
B
K erg cm- 3, erg g- 1 anisotropy constant
1
m* m effective mass
0
m g electron mass
0
M G (spontaneous) magnetization
(s)
n cm- 3 (critical) carrier concentration
(cr)
n refractive index
Landolt-Börnstein
New Series III/27C1
Ref. p. 121] 2.1.2 List of symbols and abbreviations 7
p Pa, bar, atm hydrostatic pressure
p oxygen partial pressure
O2
p m magnetic moment
B
p effective (paramagnetic) moment
eff
p magnetic moment per formula unit
FU
p ordered magnetic moment
o
p saturation, spontaneous magnetic moment
s
q, Q Å- 1, nm- 1 wavevector (Q frequently is reciprocal vector
component for neutron scattering)
R W resistance
R reflectivity
R nm radius, radial distance
R J K- 1 mol- 1 gas constant
S spin quantum number
S(Q, w ) barn sr- 1 eV- 1 atom- 1 neutron scattering response function
S m V K- 1 Seebeck coefficient, thermoelectric power
S J K- 1 mol- 1 entropy
S magnetic contribution to entropy
magn
t s, min time
T K, (cid:176) C temperature
T Curie temperature
C
T inflection point
I
T Néel temperature
N
T peak temperature (e.g. in resistivity vs. temperature
p
measurements)
T magnetic transition temperatures
C1,2,3
U eV Coulomb interaction between f electrons and core
fc
holes
U eV Coulomb interaction between f electrons
ff
u valence
V nm3 volume
x critical concentration
c
x percolation concentration
p
x, y, z fractional coordinates of atoms in the unit cell
Y (total) yield of electrons
(tot)
a, b, g deg unit cell angles
a cm- 1 absorption coefficient
G eV linewidth, half width at half maximum
D eV CEF splitting energy (mostly given as D /k in K)
B
D eV spin-orbit splitting energy
so
d oxygen deficit
e = e - ie dielectric constant
1 2
e , e real, imaginary part of dielectric constant
1 2
e eV energy of 4f level
f
e Kerr ellipticity
K
z reduced wavevector coordinate
q deg angle (scattering angle, ...)
Landolt-Börnstein
New Series III/27C1
8 2.1.2 List of symbols and abbreviations [Ref. p. 121
q deg polar Kerr rotation (angle)
K
Q K paramagnetic Curie temperature
Q K Debye temperature
D
k Å- 1 inverse correlation length
l nm, m m wavelength
m cm2 V- 1 s- 1 mobility of charge carriers
m J T- 1 Bohr magneton
B
m initial permeability
0
n Hz frequency
hn eV photon energy
n cm- 1 wave number
r W cm resistivity
s W - 1 cm- 1 electrical conductivity
s W - 1 surface conductivity
s s- 1 component of the optical conductivity tensor
ij
s emu g- 1 = G cm3 g- 1 magnetic moment per unit mass = specific
magnetization
s residual magnetization
r
s saturation, spontaneous magnetization
s
f ,j angle
c emu g- 1 = cm3 g- 1 magnetic susceptibility per gram
g
c emu mol- 1 = cm3 mol- 1 magnetic susceptibility per mole
m
c emu cm- 3 = cm3 cm- 3 volume susceptibility
v
c emu g- 1 , emu mol- 1 temperature independent magnetic susceptibility
0
c ¢, c † real, imaginary part of ac magnetic susceptibility
w s- 1 angular frequency
Abbreviations
ac alternating current
AF antiferromagnetism, antiferromagnetic
bcc body centered cubic
B-DOS Bloch density of states
BIS bremsstrahlung isochromat spectroscopy
c, cr mostly as subscript: critical
calc calculated
CEF crystal electric field
CIS constant initial state spectroscopy
CW Curie Weiss (law)
dc direct current
D diamagnetic
DOS density of states
emf electromotive force
eff effective
exp experimental
EDC electron (intensity) distribution curve
EFG electric field gradient
EPR electron paramagnetic resonance
ESR electron spin resonance
EXAFS extended X-ray absorption fine structure
fcc face centered cubic
Landolt-Börnstein
New Series III/27C1
Ref. p. 121] 2.1.2 List of symbols and abbreviations 9
F ferromagnetism, ferromagnetic
FU formula unit
FWHM full width at half maximum
HREM high resolution electron microscope
HWHM half width at half maximum
I insulator
INS inelastic neutron scattering
IS isomer shift
LAPW linearized augmented plane wave method
LDA local density approximation
LMTO linearized muffin-tin orbital
Ln lanthanide
LT low temperature
LV lower valence band
magn mostly as subscript: magnetic
max mostly as subscript: maximum
M metal (mostly transition metal)
MCW modified Curie Weiss (law)
ME Mössbauer effect
MV mixed valence
m SR muon spin resonance
ND neutron diffraction
NMR nuclear magnetic resonance
NS neutron scattering
P paramagnetism, paramagnetic
PAC perturbed angular correlation
PES photoemission spectroscopy
PEY photoelectron yield
PNSE paramagnetic neutron spin echo
PSD photon stimulated desorption
PY partial yield
Q-DOS quasiparticle density of states
QENS quasielastic neutron scattering
R rare earth element
RT room temperature
s.c. single crystal
SF spin fluctuation
SXA soft X-ray absorption
tot mostly as subscript: total
THF transferred hyperfine field
TIP temperature independent paramagnetism
TY total yield
UV upper valence band
VB valence band
WTDP weak temperature dependent paramagnet
XANES X-ray absorption near edge structural spectra
XAS X-ray absorption spectroscopy
XPS X-ray photoelectron spectroscopy
XRD X-ray diffraction
^ , || perpendicular, parallel to a crystallographic axis
(cid:1) vacancy
Landolt-Börnstein
New Series III/27C1
10 2.1.3 Survey of lanthanide oxides [Ref. p. 121
2.1.3 Survey of lanthanide oxides
Compound State Structure, magnetic and related properties Figs., Tables Ref.
La oxides
LaO High pressure technique of preparation 81LYL
r (T); T=4.2...290 K, r (4.2K)=180 Wm cm, 1 80LALD
r (290K)=270 Wm cm
La O Electronic structure: LB2, Figs. 534, 548
2 3
Theoretical analysis of spectra of the 3d core 2 87KOJB
photoemission (3d-XPS) and 2p core
photoabsorption (L -XAS) using the single
III
site Anderson model
Theoretical analysis of the hybridization effect 89IJ
between the 4f and valence band states on the
multiplet structure in 3d and 4d core X-ray
photoemission spectra (XPS)
Crystal structure: LB2, p.294, Figs. 412, 415,
422, 425, 524
Electrical and transport properties: LB2, p.294,
Figs. 486...489, 515
Thermodynamic properties: LB2, Figs.470,
522
EXAFS spectra at the La L -edge 94MCCO
III
Ce oxides
CeO Calculated electronic structure; 3 89DC
calculated valence of Ce under normal
pressure: u = 3.00 and under pressure: u = 3.26 4
Experimental valence under pressure above 86VRL
3 GPa, u = 3.3
Eigenvalues at G and X points for various 5 89DC
pressures;
DOS for various pressures; 6 89DC
NaCl-type, cubic, a=0.5089 nm, V/V (p); 7 86VRL
0
bulk modulus: B =30 GPa at p=3 GPa; 8 89LR
0
B =171(10) GPa and B '=4.8(5) at p>3 GPa
0 0
Ce O Electronic structure: LB2, Fig. 548
2 3
Theory of high energy spectroscopy 87KJON
Calculated XPS and BIS 87NKP
CeO -Ce O Experimental XPS/BIS; 9 85Al
2 2 3
thin films experimental 3d XPS/BIS 10
Theory of f-electron photoemission 87PNK
Theory of resonant photoemission 88NOK
CeO -Ce O Experimental resonant photoemission (CIS) 11 85Al
2 2 3
thin films spectra
Ce O Crystal structure: LB2, p. 295, Figs. 422, 425
2 3
Electrical and transport properties: LB2, Fig. 490
Thermodynamic properties: heat capacity LB2, Fig. 469
Melting and estimated boiling points: LB2,
Figs. 521, 522
Landolt-Börnstein
New Series III/27C1