Download citation
Download citation
link to html
Analysis of Bragg diffraction is the normal route to the structure of crystalline materials. Here we demonstrate the use of total neutron diffraction in determining the local structure in the disordered lanthanum molybdate LaMo2O5. An average structure with space-group symmetry P63/mmc accounts for the Bragg scattering and shows that the compound contains the rare Mo6O18 cluster and a unique type of Mo-Mo bonded sheet. However, this gives an incomplete picture of the structure, since it does not reveal how the sites with fractional occupancy are occupied at a local level. Two models describing possible local structures are constructed by removing symmetry elements present in the average structure. Total correlation functions, T(r), calculated from these structures, with space-group symmetry P63mc and P3m1, are compared with the experimental T(r) to show the validity of these local structures. The close relationship between the T(r)'s of the component structures gives an insight into why disorder occurs in LaMo2O5. The calculated and experimental T(r)'s for a model compound, Zn2Mo3O8, are compared to show the agreement expected from an ordered crystalline material. Remaining discrepancies between our model and the experimental T(r) give an insight into the origin of additional disorder in LaMo2O5.

Supporting information

cif

Crystallographic Information File (CIF)
Contains datablocks global, lamo2o5, zn2mo3o8

hkl

Structure factor file (CIF format)
Contains datablock lamo2o5

rtv

Rietveld powder data file (CIF format)
Contains datablock lamo2o5

hkl

Structure factor file (CIF format)
Contains datablock zn2mo3o8

rtv

Rietveld powder data file (CIF format)
Contains datablock zn2mo3o8

Computing details top

Cell refinement: TF12LS (David et al., 1992) for lamo2o5; TF12LS (David et al. (1992) for zn2mo3o8. For both compounds, program(s) used to refine structure: TF12LS (David et al., 1992).

Figures top
[Figure 1]
[Figure 2]
[Figure 3]
[Figure 4]
[Figure 5]
[Figure 6]
[Figure 7]
[Figure 8]
[Figure 9]
[Figure 10]
[Figure 11]
[Figure 12]
[Figure 13]
(lamo2o5) lanthanum molybdenum oxide top
Crystal data top
LaMo2O5Z = 12
Mr = 410.79Neutron radiation, λ = time-of-flight Å
Hexagonal, P63/mmcCell parameters from 312 reflections
Hall symbol: -P 6c 2cT = 18 K
a = 8.3680 (7) ÅPowder, black
c = 19.13687 (4) Å × × mm
V = 1160.5 (1) Å3
Data collection top
Lad tof neutron
diffractometer
h = 07
Radiation source: spallation sourcek = 04
time of flight neutron diffraction scansl = 020
312 measured reflections
Refinement top
Refinement on Inet39 parameters
Least-squares matrix: full1/[Yi]
wR(F2) = 0.048(Δ/σ)max = 0.25
312 reflections
Crystal data top
LaMo2O5V = 1160.5 (1) Å3
Mr = 410.79Z = 12
Hexagonal, P63/mmcNeutron radiation, λ = time-of-flight Å
a = 8.3680 (7) ÅT = 18 K
c = 19.13687 (4) Å × × mm
Data collection top
Lad tof neutron
diffractometer
312 measured reflections
Refinement top
wR(F2) = 0.04839 parameters
312 reflections(Δ/σ)max = 0.25
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/UeqOcc. (<1)
La10.5112 (8)0.4888 (8)0.8438 (8)0.010 (2)*0.5
La20.000000.000000.260 (1)0.010 (2)*0.5
La30.333330.666670.984 (1)0.010 (2)*0.5
La40.333330.666670.012 (1)0.010 (2)*0.5
Mo10.1066 (6)0.8934 (6)0.9428 (3)0.008 (1)*
Mo20.769 (1)0.231 (1)0.6868 (8)0.008 (9)*0.5
Mo30.561 (1)0.439 (1)0.1784 (7)0.008 (9)*0.5
O10.330 (1)0.000 (1)0.242 (7)0.0044 (9)*0.5
O20.333330.666670.2617 (7)0.0044 (9)*0.5
O30.333330.666670.752 (1)0.0044 (9)*0.5
O40.1138 (6)0.2276 (7)0.1234 (3)0.0044 (9)*
O50.560 (2)0.120 (3)0.6157 (9)0.0044 (9)*0.5
O60.454 (1)0.2268 (7)0.1130 (5)0.0044 (9)*
O70.000000.334 (1)0.000000.0044 (9)*
O100.333330.666670.399 (1)0.0044 (9)*0.5
Bond lengths (Å) top
La1—O12.4 (1)Mo1—O71.977 (8)
La1—O22.75 (2)Mo1—O62.05 (1)
La1—O33.16 (1)Mo1—O42.041 (8)
La1—O73.30 (1)Mo2—Mo22.56 (2)
La1—O42.903 (9)Mo2—Mo32.87 (1)
La1—O102.48 (2)Mo2—O11.99 (8)
La2—O12.76 (1)Mo2—O22.06 (2)
La2—O42.78 (2)Mo2—O42.09 (2)
La2—O43.09 (2)Mo2—O52.03 (2)
La3—O52.95 (3)Mo3—Mo32.65 (2)
La3—O72.804 (5)Mo3—O12.07 (8)
La3—O62.42 (2)Mo3—O32.03 (1)
La4—O62.52 (4)Mo3—O52.13 (3)
La4—O72.797 (5)Mo3—O61.98 (1)
La4—O102.16 (3)Mo1—Mo12.678 (7)
Mo1—Mo12.675 (7)Mo1—O62.05 (1)
Mo1—Mo12.678 (7)Mo3—O61.977 (8)
(zn2mo3o8) zinc molybdate top
Crystal data top
Zn2Mo3O8Z = 12
Mr = 546.6Neutron radiation, λ = time-of-flight Å
Hexagonal, P63mcCell parameters from 309 reflections
Hall symbol: P 6c -2cT = 18 K
a = 5.77322 (1) ÅPowder, black/green
c = 9.91231 (4) Å × × mm
V = 286.12 (1) Å3
Data collection top
Lad tof neutron
diffractometer
h = 06
Radiation source: spallation sourcek = 03
time of flight neutron diffraction scansl = 1313
309 measured reflections
Refinement top
Refinement on Inet29 parameters
Least-squares matrix: full1/[Yi]
wR(F2) = 0.047(Δ/σ)max = 0.001
309 reflections
Crystal data top
Zn2Mo3O8V = 286.12 (1) Å3
Mr = 546.6Z = 12
Hexagonal, P63mcNeutron radiation, λ = time-of-flight Å
a = 5.77322 (1) ÅT = 18 K
c = 9.91231 (4) Å × × mm
Data collection top
Lad tof neutron
diffractometer
309 measured reflections
Refinement top
wR(F2) = 0.04729 parameters
309 reflections
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Zn10.333330.666670.5155 (4)0.0039 (5)*
Zn20.333330.666670.9491 (6)0.0039 (5)*
Mo10.1475 (2)0.8525 (2)0.250.0032 (5)*
O10.000000.000000.8921 (5)0.0044 (3)*
O20.333330.666670.1450 (5)0.0044 (3)*
O30.4881 (3)0.5119 (3)0.3658 (3)0.0044 (3)*
O40.1662 (3)0.8338 (3)0.6334 (4)0.0044 (3)*
Bond lengths (Å) top
Mo1—Mo12.554 (2)Zn1—O32.144 (4)
Mo1—O12.039 (4)Zn1—O42.039 (3)
Mo1—O22.194 (3)Zn2—O21.942 (8)
Mo1—O32.071 (3)Zn2—O41.968 (3)
Mo1—O41.955 (4)

Experimental details

(lamo2o5)(zn2mo3o8)
Crystal data
Chemical formulaLaMo2O5Zn2Mo3O8
Mr410.79546.6
Crystal system, space groupHexagonal, P63/mmcHexagonal, P63mc
Temperature (K)1818
a, c (Å)8.3680 (7), 19.13687 (4)5.77322 (1), 9.91231 (4)
V3)1160.5 (1)286.12 (1)
Z1212
Radiation typeNeutron, λ = time-of-flight ÅNeutron, λ = time-of-flight Å
µ (mm1)??
Crystal size (mm) × × × ×
Data collection
DiffractometerLad tof neutron
diffractometer
Lad tof neutron
diffractometer
Absorption correction
No. of measured, independent and
observed (?) reflections
312, ?, ? 309, ?, ?
Rint??
Refinement
R[F2 > 2σ(F2)], wR(F2), S ?, 0.048, ? ?, 0.047, ?
No. of reflections312309
No. of parameters3929
No. of restraints??
(Δ/σ)max0.250.001
Δρmax, Δρmin (e Å3)?, ??, ?

Computer programs: TF12LS (David et al., 1992), TF12LS (David et al. (1992).

Selected bond lengths (Å) for (lamo2o5) top
La1—O12.4 (1)Mo1—O71.977 (8)
La1—O22.75 (2)Mo1—O62.05 (1)
La1—O33.16 (1)Mo1—O42.041 (8)
La1—O73.30 (1)Mo2—Mo22.56 (2)
La1—O42.903 (9)Mo2—Mo32.87 (1)
La1—O102.48 (2)Mo2—O11.99 (8)
La2—O12.76 (1)Mo2—O22.06 (2)
La2—O42.78 (2)Mo2—O42.09 (2)
La2—O43.09 (2)Mo2—O52.03 (2)
La3—O52.95 (3)Mo3—Mo32.65 (2)
La3—O72.804 (5)Mo3—O12.07 (8)
La3—O62.42 (2)Mo3—O32.03 (1)
La4—O62.52 (4)Mo3—O52.13 (3)
La4—O72.797 (5)Mo3—O61.98 (1)
La4—O102.16 (3)Mo1—Mo12.678 (7)
Mo1—Mo12.675 (7)Mo1—O62.05 (1)
Mo1—Mo12.678 (7)Mo3—O61.977 (8)
 
Follow Acta Cryst. B
Sign up for e-alerts
Follow Acta Cryst. on Twitter
Follow us on facebook
Sign up for RSS feeds