Uniaxial orientational order-disorder transitions in diammine magnesium halides, Mg(ND3)2Cl2 and Mg (ND3)2Br2, investigated by neutron diffraction

Article abstract of DOI:10.1006/jssc.2000.9028

Neutron powder diffraction on Mg(ND₃)₂Cl₂ and Mg(ND₃)₂Br₂ revealed as a function of temperature uniaxial orientational order-disorder behavior of the ND₃ groups. The crystal structures of both compounds are built up from chains of octahedra _∞¹[Mg(NH₃)₂X_4/2] with X=Cl and Br arranged in different ways relative to each other. At ambient temperatures (X=Cl) and 270 K (X=Br) the ND₃ groups are disordered with respect to a rotation about the bond Mg-N. The D atom density is well described by a fourfold split position, each D site connecting an N with an X atom: Mg(ND₃)₂Cl₂, Cmmm, a=8.1828(6) A, b=8.2007(6) A, c=3.7543(2) A, R(F²)_Bragg=5.9%; Mg(ND₃)₂Br₂, Pbam, a=5.9714(2) A, b=11.9175(3) A, c=3.98477(8) A, R(F²)_Bragg=7.9%. In both cases the c axis corresponds to the direction of the chains _∞¹[Mg(NH₃)₂X _4/2]. At low temperatures (8 K (X=Cl) and 1.5 K (X=Br)) both compounds are ordered with respect to the ND₃ groups: They are arranged antiferroelectrically on either side of the chains _∞¹[Mg(NH₃)₂X _4/2]. The symmetry is lowered compared to the situation at ambient temperatures and 270 K respectively, which involves in both cases a doubling of the orthorhombic c axis: Mg(ND₃)₂Cl₂, Ibmm, a=8.1319(3) A, b=8.1338(3) A, c=7.4410(2) A, R(F²)_Bragg=5.9%; Mg(ND₃)₂Br₂, Pnam, a=5.92837(8) A, b=11.8448(2) A, c=7.9117(1) A, R(F²)_Bragg=5.0%. Detailed evaluation of neutron diffraction data of Mg(ND₃)₂Cl₂ as a function of temperature (50 K< T< 150 K) characterizes the phase transition as continuous with T_t ≈ 135K.

Full text of DOI:10.1006/jssc.2000.9028

Journal of Solid State Chemistry 156, 487}499 (2001)
doi:10.1006/jssc.2000.9028, available online at http://www.idealibrary.com on
Uniaxial Orientational Order+Disorder Transitions in Diammine
Magnesium Halides, Mg(ND ) Cl and Mg (ND ) Br , Investigated
3
3
2
2
2
2
by Neutron Diffraction
A. Leineweber,*ꢀ-ꢀꢁ H. Jacobs,* P. Fischer,? and G. Bo
K
ttger?
*
Lehrstuhl AC I, Fachbereich Chemie der Universita
K
t, 44221 Dortmund, Germany; - Max-Planck-Institut fuK r Metallforschung, Seestra}e 92, 70174 Stuttgart,
Germany; and ?Labor fu
K
r Neutronenstreuung, ETHZ & PSI, 5232 Villigen PSI, Switzerland
Received September 11, 2000; accepted November 6, 2000
di!erent structure types, the Cd(NH ) Cl type (Cmmm (2))
ꢃ ꢄ
ꢄ
Neutron powder di4raction on Mg(ND ) Cl
2
and for X"Cl and the Mg(NH ) Br type (Pbam (1)) for
ꢃ ꢄ ꢄ
3
2
Mg(ND ) Br revealed as a function of temperature uniaxial
3
2
2
X"Br, I. Both types contain parallel octahedral chains
ꢁ [Mg(NH ) X ], which run along the direction [001]
orientational order+disorder behavior of the ND groups. The
3
ꢂ
ꢃ ꢄ ꢅꢆꢄ
crystal structures of both compounds are built up from chains of
(
Fig. 1) and which are arranged in di!erent ways relative to
1
octahedra [Mg(NH ) X ] with X ؍ Cl and Br arranged in
3
2
4/2
ꢂ
each other within the plane (001) (Fig. 2).
di4erent ways relative to each other. At ambient temperatures
X ؍ Cl) and 270 K (X ؍ Br) the ND groups are disordered with
The two crystal structure types found for Mg(NH ) X
(
ꢃ ꢄ ꢄ
3
with X"Cl, Br, I were reported for several analogous
respect to a rotation about the bond Mg+N. The D atom density
is well described by a fourfold split position, each D 99site::
connecting an N with an X atom: Mg(ND ) Cl , Cmmm,
compounds M(NH ) X (M"Mn, Fe, Co, Ni; X"Cl,
ꢃ ꢄ ꢄ
Br, I) in a preliminary paper (3). More detailed investiga-
3
2
2
2
a ؍ 8.1828(6) A
s
, b ؍ 8.2007(6) A
s
, c ؍ 3.7543(2) A
s
, R(F )
؍
tions on Ni(NH ) X with X"Cl, Br, I (4) established
Bragg
ꢃ ꢄ ꢄ
5
.9%; Mg(ND ) Br , Pbam, a ؍ 5.9714(2) A
s
, b ؍ 11.9175(3) A
s
,
that for X"Br, I the Mg(NH ) Br type structure is the
3
2
2
ꢃ ꢄ
ꢄ
2
c ؍ 3.98477(8) A
s
, R(F )
؍ 7.9%. In both cases the c axis stable modi"cation; however, metastable phases of the
Bragg
1
corresponds to the direction of the chains [Mg(NH ) X ]. At
low temperatures (8 K (X ؍ Cl) and 1.5 K (X ؍ Br)) both com-
3
2
4/2
Cd(NH ) Cl type structure isotypic to Ni(NH ) Cl can
ꢃ ꢄ ꢃ ꢄ
also be obtained.
ꢂ
ꢄ
ꢄ
pounds are ordered with respect to the ND groups: They are
3
One interesting aspect of the structures of these ammines
arranged antiferroelectrically on either side of the chains
is the behavior of the NH molecules with respect to a rota-
1
[
Mg(NH ) X ]. The symmetry is lowered compared to
ꢃ
3
2
4/2
ꢂ
tion about the coordinative bond M}N. Many studies con-
the situation at ambient temperatures and 270 K respectively,
which involves in both cases a doubling of the orthorhombic
cerning this topic are focused on hexaammines M(NH ) X
ꢃ ꢇ ꢄ
with M"Ca, Mn, Fe, Co, Ni, Zn, Cd, and X"Cl, Br, I.
c axis: Mg(ND ) Cl , Ibmm, a ؍ 8.1319(3) A
s
, b ؍ 8.1338(3) A
s
,
3
2
2
2
The crystal structures of these compounds are based on
c ؍ 7.4410(2) A
s
, R(F )
Bragg
؍ 5.9%; Mg(ND ) Br , Pnam, a ؍
3 2
2
2
5
.92837(8) A, b؍11.8448(2) A
s
s
, c؍7.9117(1) A, R(F )
s
؍5.0%. a CaF /anti-K PtCl type arrangement (5, 6). At ambient
Bragg
ꢄ
ꢄ
ꢇ
Detailed evaluation of neutron di4raction data of Mg(ND ) Cl
temperatures X-ray and neutron di!raction indicate dis-
3
2
2
as a function of temperature (50 K< T < 150 K) characterizes order of the ammonia groups in such hexaammines (7}9).
the phase transition as continuous with T +135 K.
( 2001
t
The dynamics of the reorientational processes were studied
using various methods (10). Going to low temperatures,
order}disorder phase transitions with respect to an ordering
Academic Press
Key Words: diammine magnesium halides; ammoniates of mag-
nesium halides; order+disorder transitions; dynamical disorder of
ammonia ligands; neutron di4raction.
of the NH groups were detected in many cases (e.g., (11)).
ꢃ
Recently, the crystal structures of low-temperature modi"-
cation of Ni(ND ) X with X"Cl, Br, I were determined
ꢃ ꢇ ꢄ
ꢃ
in which the ND groups are ordered (12).
The dynamics and ordering behavior is more complicated
INTRODUCTION
if complex anions are involved, e.g., X"NO , NO , BF ,
Recently, we reported on preparation and crystal struc-
tures of the diammine magnesium halides Mg(NH ) X
with X"Cl, Br, I (1). These compounds crystallize in two
ꢃ
ꢄ
ꢅ
and ClO . In such compounds both order}disorder phe-
ꢅ
ꢃ ꢄ ꢄ nomena concerning a uniaxial rotation of the ND groups
ꢃ
about the bonds M}N as well as a more or less free rotation
of the complex anions occur. Examples for structural
studies on phase transitions in such systems are, e.g., on
ꢁ
To whom correspondence should be addressed. Fax: #49 (0)711 2095
4
20. E-mail: leineweber@mf.mpi-stuttgart.mpg.de.
4
87
0
022-4596/01 $35.00
Copyright ( 2001 by Academic Press
All rights of reproduction in any form reserved.

4
88
LEINEWEBER ET AL.
elements. Therefore, this method is well suited to provide
detailed structural data about the order}disorder
phenomena with respect to ND in these compounds.
ꢃ
EXPERIMENTAL
Preparation of Mg(ND ) Cl and Mg(ND ) Br
3
2
2
3 2
2
MgCl , MgBr as well as their ammines are sensitive
ꢄ
ꢄ
toward moist air and were handled under inert gas condi-
tions (16).
FIG. 1. Chains ꢁ[Mg(NH ) X ] (with X"Cl, Br, I; H omitted) of
ꢃ ꢄ ꢅꢆꢄ
edge-sharing octahedra as main building units of Mg(NH ) X (X"Cl,
ꢃ ꢄ
ꢂ
MgCl (approximate batch size: 1.0 g, Alfa, 99.9%) was
ꢄ
ꢄ
Br, I).
placed into a glass tube cooled by ice water. It was reacted
with an excess of ND ('99%, Cambridge Isotope Labor-
ꢃ
atories) at ambient temperatures, yielding Mg(ND ) Cl .
ꢃ ꢇ
ꢄ
Ni(ND ) (NO ) (13), and, not belonging to the large group The tube was then attached to dynamic vacuum (p(0.1
ꢃ ꢇ ꢃ ꢄ
of K PtCl type hexaammines, on Ag(ND ) NO (14, 15). Torr) and slowly heated to 1203C. This procedure removed
ꢃ ꢃ
X-ray powder and single-crystal crystal data on ꢄ of the ammonia from the solid leading to Mg(ND ) Cl .
ꢃ ꢄ
Anhydrous MgBr was obtained by reacting Mg turnings
ꢄ
ꢇ
ꢃ
ꢃ
ꢄ
Mg(NH ) X with X"Cl, Br, I (1) suggest that the NH
ꢃ ꢄ ꢄ
ꢃ
ꢄ
groups are disordered with respect to a rotation about the (Ventron, '99%) with NH Br (Aldrich, '99%) in the
bond Mg}N at ambient temperatures. However, experi- ratio 1:2.1 in a steel autoclave (17). The product (composed
ꢅ
mental evidence for the distribution of H in the of Mg(NH ) Br and NH Br) was placed into a glass tube
ꢃ ꢄ
ꢄ
ꢅ
Mg(NH ) X compounds is relatively weak because of the attached to dynamic vacuum (p(0.1 Torr). The tube was
ꢃ ꢄ ꢄ
small scattering power of H for X-rays. Therefore, the re- slowly heated to a maximum temperature of 3503C. The
sults were regarded as preliminary.
resulting product was MgBr . It was characterized by X-ray
ꢄ
Now, we report on temperature-dependent neutron di!raction and IR spectroscopy. A part of the anhydrous
di!raction data on Mg(ND ) Cl and Mg(ND ) Br . In bromide was reacted with ND to yield Mg(ND ) Br ,
ꢃ ꢄ
ꢄ
ꢃ ꢄ
ꢄ
ꢃ
ꢃ ꢇ
ꢄ
neutron di!raction the scattering power of D is within the as described above for the corresponding chloride.
same order of magnitude as that of the other constituting Mg(ND ) Br and MgBr were mixed in a molar ratio of
ꢃ ꢇ
ꢄ
ꢄ
FIG. 2. Crystal structures of Mg(NH ) Cl (left) and Mg(NH ) Br (right) projected onto (001) showing the relation to a CsCl type structure: N and
ꢃ ꢄ
ꢄ
ꢃ ꢄ
ꢄ
Cl occupy the Cs and Cl sites. Double-CsCl units are formed by connecting two CsCl units by an Mg atom on a common face. In such a double-CsCl unit
an octahedron as part of a chain ꢁ [Mg(NH ) X ] running along [001] is located. The structures of Mg(NH ) Cl and Mg(NH ) Br (as well as
ꢂ
ꢃ ꢄ ꢅꢆꢄ
ꢃ ꢄ
ꢄ
ꢃ ꢄ
ꢄ
Mg(NH ) I ) di!er by the mutual arrangement of the Double-CsCl units and, therefore, of the chains ꢁ[Mg(NH ) X ].
ꢃ ꢄ ꢄ ꢂ ꢃ ꢄ ꢅꢆꢄ

UNIAXIAL ORIENTATIONAL ORDER}DISORDER TRANSITIONS
489
TABLE 1
Technical Data of Neutron Powder Di4raction and Rietveld Re5nement of Mg(ND ) Cl
3
2
2
Temperature of data collection
Ambient temperatures
8 K
50}150 K
Formula
Mg(ND ) Cl
134.32 g/mol
HRPT (SINQ, PSI, Villigen, CH)
ꢃ ꢄ
ꢄ
Formula weight
Di!ractometer
Wave length
1.4930 A
s
1.8857 A
s
2
h Range used
103}1653
for Rietveld re"nement
2
h Step width
0.053
0.103
No. of pro"le parameters
No. of background parameters
Space group
5
8
6
8
5
8
Cmmm
Ibmm (i.e., Imma)
Lattice parameters?
a"8.1828(6) A
b"8.2007(6) A
c"3.7543(2) A
s
s
a"8.1319(3) A
b"8.1338(3) A
c"7.4410(2) A
2;3.7205(1) A
492.17(3) Aꢃ, 4
23
350
0.063/0.050
0.059
s
s
s
see Fig. 10
s
"
s
Cell volume, Z
251.93(3) A
s
ꢃ, 2
s
Structural parameters
No. of independent re#ections
wR /R
14
204
0.041/0.039
0.090
.
.
R(Fꢄ) (Fꢄ5p(Fꢄ))
?
Standard deviations do not take into account the uncertainty of the wavelength.
1
:2, "lled in a steel autoclave, and heated to 3003C for 3 days (CuKa , FR 552, Enraf Nonius, Delft (NL)), and tested by
ꢁ
to yield Mg(ND ) Br .
IR spectroscopy for the presence of high amounts of H.
ꢃ ꢄ
ꢄ
Care must be taken during the evacuating procedures to
avoid a vigorous decomposition of the solids which may
lead to a contamination of the vacuum line by the very "ne
powders. A glass fritted disc between sample and vacuum
line may prevent this.
Neutron Diwraction and Rietveld Rexnement
Powdered samples were "lled in vanadium cans (diameter
of 8 mm) closed with gold sealings.
The "nal products, Mg(ND ) Cl and Mg(ND ) Br ,
were free from impurities as to Guinier powder photographs the di!ractometer HRPT (18) installed at the spallation
Neutron di!raction on Mg(ND ) Cl was performed on
ꢃ ꢄ
ꢄ
ꢃ ꢄ
ꢄ
ꢃ ꢄ ꢄ
TABLE 2
Positional and Displacement Parameters for Mg(NH ) Cl
3
2
2
Atom
Wycko!-site
x
y
z
f
s
;'1-, ;%ꢀ (Aꢄ)
-
!!
Ambient temperatures, Cmmm
Mg
Cl
N
2a
4h
4i
0
0
0
0
ꢁ
ꢄ
0
1
1
1
0.025(2)
0.0203(7)
0.0275(9)
0.088@
0.2140(3)
0
0.0755(6)
0.2591(4)
0.3094(4)
D
16r
0.1626(9)
ꢃ
ꢅ
8
K, Ibmm
Mg
4e?
8h
8i
8i
16j
0.0041(9)
0.2152(2)
0.0125(3)
0
0
ꢁ
1
1
1
1
1
0.006(1)
0.0034(4)
0.0039(4)
0.032@
ꢅ
Cl
N
D(a)
D(b)
!0.0060(3)
0.2626(3)
0.3206(7)
0.3079(4)
ꢁ
ꢅ
!0.0926(6)
0.0746(4)
ꢁ
ꢅ
0.3562(3)
0.026@
?
These Wycko! site labels refer to the standard setting Imma. The origin for the high temperature phase is (0 0 ꢁ) with respect to the coordinates of the
low-temperature phase.
ꢅ
@
The coe$cients of the ;GH
tensor are given in Table 5. ; "ꢁ Tr (; ).
%ꢀ
ꢃ
GH

4
90
LEINEWEBER ET AL.
TABLE 3
a wide range of d spacings. Whereas at ambient temper-
Technical Data of Neutron Powder Di4raction and Rietveld
Re5nement of Mg(ND )Br
atures the high-intensity mode was used, higher resolution
3
2
was obtained for ¹"8 K by primary collimation a "12ꢀ
ꢁ
and secondary collimation a +24ꢀ. Faster data acquisition
ꢄ
Temperature of data collection:
270 K
1.5 K
was desired for the characterization of the phase transition
in the range 8 K4¹4150 K. Here, j"1.8857 A
s
and
Formula
Mg(ND ) Br
ꢃ ꢄ
224.24 g/mol
ꢄ
a stepwidth of 0.13 was used. The data were corrected for
Formula weight
Di!ractometer
Wavelength
D1A (ILL, Grenoble (F))
1.9114 A
absorption by measuring the transmission of the sample at
s
j"1.8857 A
calculated.
Neutron di!raction on Mg(ND ) Br was performed on
s
. From this the absorption for 1.4930 A was
s
2
2
h Range
h Step width
103}1553
0.053
No. of pro"le parameters
No. of background parameters
Space group
Lattice
parameters?
6
4
6
5
ꢃ ꢄ
ꢄ
the di!ractometer D1A (19) installed at the ILL (Grenoble
Pbam
Pnam (i.e., Pnma)
a"5.92837(8) A
b"11.8448(2) A
c"7.9117(1) A
2;3.95586(7) A
555.56(2) Aꢃ, 4
(F)) with a wavelength of 1.9114 A. Low temperatures were
s
a"5.9714(2) A
b"11.9175(3) A
c"3.98477(8) A
s
s
s
generated by a standard Orange Cryostat. For these data no
absorption correction was applied.
s
s
s
Rietveld re"nement was performed using the GSAS pack-
age of programs (20). Neutron scattering lengths used were
0.6674 fm for D, 0.930 fm for N, 0.5375 fm for Mg, 0.9579 fm
for Cl, and 0.679 fm for Br. The background of the di!rac-
tion patterns was "tted using shifted Chebichev polynoms.
The re#ection pro"les were "tted by an exponential Pseudo-
Voigt convolution as implemented in the GSAS package.
"
s
Cell volume, Z
283.57(1) A
s
ꢃ, 2
s
Structural parameters
No. of independent re#ections
wR /R
25
194
43
349
0.051/0.039
0.079
0.048/0.037
0.050
.
.
R (Fꢄ) (Fꢄ5p (Fꢄ))
?
Standard deviations do not take into account the uncertainty of the
wavelength.
RESULTS AND DISCUSSION
Evaluation of Neutron Diwraction Data Taken
at Ambient Temperature
neutron source SINQ at the PSI (Villigen (CH)). Low tem-
peratures were achieved by a closed-cycle helium cryostat.
Neutron di!raction data of Mg(ND ) Cl taken at ambi-
For the detailed structure analyses performed at 8 K and ent temperatures as well as of Mg(ND ) Br taken at 270 K
ꢃ ꢄ
ambient temperatures di!raction data were collected with show only re#ections which can totally be indexed with the
ꢃ ꢄ
ꢄ
ꢄ
j"1.4930 A
s
and a stepwidth of 0.053 in order to have unit cell parameters obtained from the X-ray di!raction
TABLE 4
Positional and Displacement Parameters for Mg(NH ) Br
3
2
2
Atom
Wycko! site
x
y
z
f
s
;'1-, ;%ꢀ (Aꢄ)
-
!!
2
70 K, Pbam
Mg
Br
N
D
Dꢀ
2a
4h
4g
8i
0
0
0
ꢁ
ꢄ
0
1
1
1
ꢃ
ꢅ
ꢃ
ꢅ
0.013(1)
0.0133(9)
0.0230(9)
0.073@
0.2776(6)
0.2497(7)
0.365(1)
0.228(1)
0.3858(4)
0.1250(4)
0.1120(5)
0.1875(6)
0.146(2)
0.154(2)
8i
0.100@
1
.5 K, Pnam
Mg
4c?
8d
4c
4c
4d
4c
4c
8d
0.002(1)
0.2788(4)
0.2575(6)
0.213(1)
0.3616(8)
0.7476(7)
0.593(1)
0.758(1)
0.0006(6)
0.3872(2)
0.1224(4)
0.2014(7)
0.1163(5)
0.8735(4)
0.8954(8)
0.8190(4)
ꢁ
ꢅ
1
1
1
1
1
1
1
1
0.003(1)
!0.0039(5)
!0.0004(8)
0.050@
Br
N(1)
0.0005(4)
ꢁ
ꢅ
D(1a)
D(1b)
N(2)
D(2a)
D(2b)
ꢁ
ꢅ
0.1541(6)
0.038@
ꢁ
0.0045(8)
0.061@
ꢅ
ꢁ
ꢅ
0.1553(6)
0.041@
?
These Wycko! site labels refer to the standard setting Pnma. The origin for the high-temperature phase is (0 0 ꢁ) with respect to the coordinates of the
low-temperature phase.
ꢅ
@
The coe$cients of the ;GH
tensor are given in Table 5. ; "ꢁ Tr (; ).
%ꢀ
ꢃ
GH

UNIAXIAL ORIENTATIONAL ORDER}DISORDER TRANSITIONS
491
FIG. 3. Observed (#) and calculated (!) neutron di!raction pro"les (top) and di!erence curve (bottom) of Mg(NH ) Cl (j"1.4930 A
s
): ambient
ꢃ ꢄ
ꢄ
temperatures (a) and 8 K (b). Markers indicate positions of re#ections of the high- and low-temperature phase, respectively. (c) An enlarged part of
b) depicting some superstructure re#ections and showing in a box the part of the patterns with the strong pair of superstructure re#ections (235)#(325)
used to analyze the temperature-dependent evolution of the superstructure using j"1.8857 A data.
(
s
data (1). Rietveld re"nements were performed using struc- di!raction studies on Ni(ND ) Br at ambient temperatures
ꢃ ꢇ
tural models with a disordered distribution of three (9). That compound belongs to the CaF /anti-K PtCl
ꢄ
ꢄ
ꢄ
ꢇ
D atoms over four sites, as used for the evaluation of the structure type (space group Fm 3ꢁ m) with Ca replaced by the
experimental data of Ref. (1). All positional parameters, octahedral complex cation. The ND molecules point with
ꢃ
isotropic thermal parameters for Mg, Cl/Br, and N as well their D atoms toward a square of Br atoms. The site sym-
as anisotropic thermal parameters for the D sites were metry of N (C ) is incompatible with the threefold sym-
ꢅJ
re"ned successfully. Technical data of the measurements metry of the ND molecules (C ). This incompatibility is
ꢃ
ꢃJ
and the re"nements are summarized in Tables 1 and 3, the overcome by a rotation of the ND molecules about the axis
ꢃ
structural parameters are given in Tables 2 and 4.
Ni}N. The rotation is accompanied by a translational
Figures 3a and 4a show the di!raction patterns includ- motion of the ND molecules, i.e. the N atoms are slightly
ꢃ
ing Rietveld "ts. Figures 5a and 5b give the octahedra displaced from the fourfold axis (21). Time-averaging "nally
Mg(ND ) Cl and Mg(ND ) Br as building units of yields the observed distribution of D with four maxima of
ꢃ ꢄ
ꢅ
ꢃ ꢄ
ꢅ
the chains ꢁ[Mg(ND ) X ] (Figs. 6a and 6b). For density pointing toward the Br atoms.
ꢃ ꢄ ꢅꢆꢄ
Mg(ND ) Cl we additionally illustrate the distribution of
ꢃ ꢄ
ꢂ
ꢄ
The local environment of ND by X in Mg(ND ) Cl and
ꢃ ꢄ
ꢃ
ꢄ
D by Fourier sections (Fꢁ"ꢂ) through the plane formed by Mg(ND ) Br is similar to that in Ni(ND ) Br . The posit-
ꢃ ꢄ
ꢄ
ꢃ ꢇ
ꢄ
the D atoms (Fig. 7).
The distribution of D in Mg(ND ) Cl and Mg(ND ) Br
ively charged part of the ND dipole is directed toward
ꢃ
a rectangle of X anions which is close to a square (Figs. 5a
ꢃ ꢄ
ꢄ
ꢃ ꢄ
ꢄ
is similar to that obtained by detailed single-crystal neutron and 5b). The site symmetry of N is C and C for
ꢄT
1

4
92
LEINEWEBER ET AL.
s
FIG. 4. Observed (#) and calculated (!) neutron di!raction pro"les (top) and di!erence curve (bottom) of Mg(NH ) Br (j"1.9114 A): 270 K (a)
ꢃ ꢄ
ꢄ
and 1.5 K (b). Markers indicate positions of re#ections of the high- and low-temperature phase, respectively. (c) An enlarged part of (b) depicting some
superstructure re#ections.
Mg(ND ) Cl and Mg(ND ) Br , respectively. C is again c axes of either unit cell. However, structure solution is not
ꢃ ꢄ
ꢄ
ꢃ ꢄ
ꢄ
ꢄT
incompatible with C ; however, C is compatible. Actually, straightforward because many of the superstructure
ꢃT
1
for Mg(ND ) Br ordering is possible without a reduction re#ections are quite weak. Furthermore, an unambiguous
of symmetry. Nevertheless, the distribution of D is dis- indexing of the new re#ections in not possible as both
ꢃ ꢄ
ꢄ
ordered as in Mg(ND ) Cl . The decisive reasons for dis- crystal structure types under consideration have pseudotet-
ꢃ ꢄ
ꢄ
order at ambient temperatures are interatomic potentials ragonal cell dimensions (a+b and 2 a+b, respectively;
and entropy. In fact, it is shown in the next section that the compare Tables 1 and 3, discussed in detail in Ref. (1)).
most simple type of ordering of ND in Mg(ND ) Br , Therefore, we constructed di!erent ordered starting models
ꢃ
ꢃ ꢄ
ꢄ
which would preserve the Pbam symmetry, is not formed at to be tested by Rietveld re"nement.
low temperatures.
As discussed previously for the hexaammines (9, 21, 22),
the particular orientation of the ammonia groups should
mainly depend on interactions H2X (ammonia group
with neighboring anions) and H 2 H (between di!erent
ammonia groups). As in the hexaammines one has to expect
Evaluation of Neutron Diwraction Data
at Low Temperatures
The di!raction patterns of Mg(ND ) Cl and of frustration for the interactions H2X as these cannot fully
ꢃ ꢄ
Mg(ND ) Br taken at 8 and 1.5 K, respectively, show be optimized due to the incompatibility of the threefold
ꢃ ꢄ
additional re#ections, compared to the situation at 295 and symmetry of the ammonia group and the approximate four-
ꢄ
ꢄ
2
70 K, respectively, (Figs. 3 and 4). In both cases, the super- fold symmetry of the environment of the ammonia group by
structure re#ections can be attributed to a doubling of the X (see Fig. 5). The interactions H2H are expected to be

UNIAXIAL ORIENTATIONAL ORDER}DISORDER TRANSITIONS
493
FIG. 5. Units Mg(ND ) Cl (a and c) and Mg(ND ) Br (b and d) as constituents of the chains ꢁ [Mg(ND ) X ] with ND molecules pointing
ꢃ ꢄ ꢃ ꢄ ꢃ ꢄ ꢅꢆꢄ
toward rectangles of X. Only for the D atoms information about displacement parameters is given (50% probability).
ꢅ
ꢅ
ꢂ
ꢃ
repulsive. In Mg(ND ) Cl and Mg(ND ) Br they should are equivalent, four di!erent superstructures can be con-
ꢃ ꢄ
ꢄ
ꢃ ꢄ
ꢄ
be strongest between ammonia groups which are adjacent structed for each of the two structure types under considera-
on one side of an octahedral chain. Here, the distance tion. This number arises from two possibilities how to
N 2 N (which carry the interacting D atoms) is closest and arrange two ND groups bonded to one Mg, which we call
ꢃ
corresponds the length of the cell parameter of the dis- eclipsed and staggered (analogous to the conformations of
ordered phase, c . As the superstructure formation involves ethane molecules) and, additionally, from two possibilities
ꢈ
a doubling of c it is suggested that the ND molecules of orientation for the chains with the Mg atoms at (ꢁ ꢁ z)
ꢈ
ꢃ
ꢄ ꢄ
order in the way shown in Fig. 8. Such an &&antiferroelectric'' compared to those at (0 0 z).
left}right}left ordering of the D(a) atoms on the mirror Figures 9 and 10 show schematically the four di!erent
planes2/m should be favored in terms of electrostatic superstructures derived for Mg(ND ) Cl and of
ꢃ ꢄ
interactions. In contrast, a &&ferroelectric'' right}right}right Mg(ND ) Br , respectively. The space groups of the super-
ꢃ ꢄ
identical to left}left}left) ordering which retains the transla- structures are subgroups of the order 2, resulting from
ꢄ
ꢄ
(
tion c , should be avoided. An arrangement like this could doubling of c of the space groups Cmmm and Pbam of the
ꢈ
ꢈ
be constructed for Mg(ND ) Br in its original unit cell with high-temperature phase, respectively (23). Note that the
ꢃ ꢄ
ꢄ
Pbam symmetry as mentioned in the section above.
Favoring a doubling of c and the ordering principle as to
Fig. 8 and assuming, furthermore, that all octahedral chains yields convincing "ts in Ibmm (Imma) for Mg(ND ) Cl and
mirror planes
/m are preserved in all structural models.
2
Testing these structure models in Rietveld re"nement
ꢈ
ꢃ ꢄ
ꢄ

4
94
LEINEWEBER ET AL.
FIG. 6. Chains of octahedra ꢁ [Mg(ND ) X ] present in Mg(ND ) Cl (a and c) and Mg(ND ) Br (b and d). For the c and d (low-temperature
ꢂ
ꢃ ꢄ ꢅꢆꢄ
ꢃ ꢄ
ꢄ
ꢃ ꢄ
ꢄ
phases) the arrows indicate the displacements of the ND molecules. Only for the D atoms information about displacement parameters is given (50%
probability).
ꢃ
in Pnam (Pnma) for Mg(ND ) Br . The low-temperature
phase structure model of the chloride leads to two di!erent which obviously involves hydrogen bonding, whereas the
kinds of D sites, labeled D(a), situated on the mirror planes D(a) atoms point between two X atoms (Figs. 5c and 5d).
The D(b) atoms are directed approximately to X atoms,
ꢃ ꢄ
ꢄ
/
m, and D(b) (Fig. 8). For Mg(ND ) Br the &&left'' and The nuclear scattering density of D is signi"cantly more
2
ꢃ ꢄ
ꢄ
&
&right'' orientation of the ND molecules are inequivalent in smeared out than that of the Mg, X, and N atoms. This is
symmetry because of the speci"c mutual arrangement of the exhibited by the thermal parameters (Tables 2 and 4). It
chains ꢁ [Mg(ND ) X ]. Therefore, one yields four D sites cannot be said from these data whether this is due to
D(1a), D(1b), D(2a), D(2b)) and consequently two N sites thermal motion of D in a very soft potential or due to
N(1) and N(2)). a superposition of two or more minima of energy of a single
Details and results of Rietveld re"nements are given in ND molecule in its local environment. For the chloride the
ꢃ
ꢂ
ꢃ ꢄ ꢅꢆꢄ
(
(
ꢃ
Tables 1}5, bond lengths and angles are listed in Table 6. form of the ellipsoids is compatible with mainly librational
Figures. 5c and 5d show the octahedra Mg(ND ) X dis- motion of the D atoms. For the bromide the re"nement of
ꢃ ꢄ ꢅ
playing thermal ellipsoids for D. A Fourier section (F
)
;
components of the D atoms leads to a signi"cant
ꢁ"ꢂ
GH
through the D atoms of an ND molecule of the low- improvement of residuals. However, details of the ; ten-
temperature phase of Mg(ND ) Cl is plotted in Fig. 7.
ꢃ
GH
sors as well as the values of the other thermal parameters
ꢃ ꢄ
ꢄ

UNIAXIAL ORIENTATIONAL ORDER}DISORDER TRANSITIONS
495
FIG. 7. Fourier sections (F ) for Mg(ND ) Cl for ambient temper-
ꢁ
"ꢂ
ꢃ ꢄ ꢄ
atures (a) and 8 K (b) data. Each time six equidistant steps of nuclear
scattering density were chosen between 0 and the maximum density. For
(
a) the section shows the a plane (010) at a height y"0.3094, for (b) at
y"0.3143.
should be treated with care due to the high minimum
d spacings of di!raction data compared to those measured
for the chloride.
FIG. 9. Schematic illustration of possible ordering types of
Mg(ND ) Cl with an &&antiferroelectric'' arrangement of the ND molecu-
ꢃ ꢄ
ꢄ
ꢃ
les and one type of chain, ꢁ [Mg(ND ) Cl ]. Compare Fig. 8 for the
ꢂ
ꢃ ꢄ ꢅꢆꢄ
meaning of the pictograms. Within an unit cell there is a second plane (001)
with all D(a) on the other side of N.
The ordering of the ND molecules leads to additional
ꢃ
atomic displacements relative to the positions in the high
temperature phases. The most prominent feature of both
low temperature phases is a bending of the angles N}Mg}N
from 1803 to 1763 which accompanies the eclipsed confor-
mation of the D N}Mg}ND units (Figs. 5c and 5d). The
ꢃ
ꢃ
bending leads to a displacement of N of about 0.07 A
s
from its mean position of the disordered phase and
further reduces the repulsive electrostatic interactions of
FIG. 8. &&Antiferroelectric'' model for ordering of ND groups on one adjacent ND molecules of one ordered side of chain
ꢃ
ꢃ
ꢃ ꢄ ꢅꢆꢄ
single side of a chain ꢁ [Mg(ND ) X ] in the LT phase of Mg(ND ) Cl
ꢃ ꢄ ꢅꢆꢄ ꢃ ꢄ
and Mg(ND ) Br . It can be regarded to be energetically favored in terms
ꢁ
[Mg(ND ) X ] along [001]. Furthermore, these dis-
ꢂ
ꢄ
ꢂ
ꢃ ꢄ
ꢄ
placements move the D(b) atoms of the ND molecules
of repulsive interactions D 2 D and implies a doubling of the c axis. On
the left one looks on a chain viewed along the axes N}Mg}N. Cl is not
shown. The ND groups are oriented in such a way that one sort of
ꢃ
toward pairs of X atoms. Thereby, the hydrogen bonds
N}D(b)2X become more e$cient.
ꢃ
D (called D(a)) lies on mirror planes .../m. These are used to represent the
The displacements of the N atoms in the low-temperature
phase of the diammines are of similar magnitude as the
displacements in the disordered phases of hexaammines (21)
derived from an analysis of the coupling of rotational and
translational motion of ammonia. One might imagine that
similar displacements may also be present in the high-
ordering on the right. The three pictograms show the Mg, N, and D(a)
atoms on the three single-mirror planes .../m with additional Cl atoms
above and below the mirror planes (view along [001]). For the &&antifer-
roelectric'' ordering scheme, the orientation of the ND groups in one
ꢃ
single-mirror plane is su$cient to represent the ordering of the whole
structure.

4
96
LEINEWEBER ET AL.
FIG. 10. Schematic illustration of possible ordering types of Mg(ND ) Br in a way as done in Fig. 9 for Mg(ND ) Cl .
ꢃ ꢄ
ꢄ
ꢃ ꢄ ꢄ
temperature phases of the diammines. Evidence for this smaller (Figs. 5d and 6d). This is attributed to the fact, that,
comes from the fact that the ; values for N in the in contrast to the situation in the chloride, one D(a) and two
*
ꢂꢁ
high-temperature phases are larger than those of Mg and D(b) atoms point to each pair Br 2 Br.
X (see Tables 2 and 4). Given the &&antiferroelectric'' arrangement of ND mol-
Furthermore, in the low-temperature phase of the chlor- ecules on one side of the chains ꢁ[Mg(ND ) X ], it is
ꢃ
ꢂ
ꢃ ꢄ ꢅꢆꢄ
ide the Cl atoms are displaced from the planes (001) at z"0 di$cult to discuss the reasons why the particular observed
and 0.5 which are mirror planes in the high-temperature low-temperature phase structures are formed out of the four
phase at z"0.5 (compare Table 2). This yields short models constructed for the chloride and the bromide,
(
s s
3.63 A) and long (3.81 A) distances Cl 2 Cl along [001]. respectively (Figs. 9 and 10). Most probably there is no
The short distance results when two D(a) atoms from di!er- general rule behind the fact that in both cases an eclipsed
ent ND molecules (only one of these ND molecules is conformation of the D N}Mg}ND units is present. Strik-
ꢃ
ꢃ
ꢃ
ꢃ
shown in Fig. 5c) point to a pair Cl 2 Cl, the long ones ingly, in the low-temperature modi"cations of Ni(ND ) X
ꢃ ꢇ ꢄ
when 2;2 D(b) are involved. The consequences for the with X"Cl, Br, I (12) which all exhibit di!erent ordering
geometry of a single chain are illustrated in Fig. 6c. For the patterns of ND , always an eclipsed conformation is pres-
ꢃ
bromide the displacements of Br along [001] are much ent. However, in a medium temperature modi"cation of

UNIAXIAL ORIENTATIONAL ORDER}DISORDER TRANSITIONS
497
TABLE 5
a
Components of the U Tensors of the D-sites in Mg(ND ) Cl and Mg(ND ) Br
ij
3 2
2
3 2
2
Atom
;ꢁꢁ (A
s
ꢄ)
;ꢄꢄ (A
s
ꢄ)
;ꢃꢃ (A
s
ꢄ)
;ꢁꢄ (A
s
ꢄ)
;ꢁꢃ (A
s
ꢄ)
s
;ꢄꢃ (Aꢄ)
Mg(ND ) Cl
D
295 K
8 K
ꢃ ꢄ
ꢄ
0.098(5)
0.060(3)
0.107(3)
!0.007(2)
!0.042(3)
!0.026(2)
Mg(ND ) Cl
D(a)
ꢃ ꢄ
ꢄ
0.032(3)
0.041(2)
0.028(3)
0.019(2)
0.037(3)
0.017(1)
0.007(2)
!0.002(1)
0
0
D(b)
Mg(NH ) Br
D(1)
D(2)
Mg(NH ) Br
D(1a)
D(1b)
D(2a)
D(2b)
!0.010(1)
!0.008(1)
270 K
1.5 K
ꢃ ꢄ
ꢄ
0.055(4)
0.083(5)
0.071(4)
0.089(5)
0.093(5)
0.128(7)
!0.030(3)
!0.023(7)
!0.028(3)
0.007(4)
!0.060(5)
0.042(5)
ꢃ ꢄ
ꢄ
0.038(5)
0.039(3)
0.019(4)
0.051(3)
0.034(4)
0.047(3)
0.027(4)
0.030(2)
0.078(7)
0.027(3)
0.14(1)
!0.021(4)
!0.012(3)
!0.015(3)
!0.009(3)
0
0
0.010(2)
0
0.011(3)
!0.013(3)
0
!0.007(3)
0.041(3)
?
The anisotropic thermal displacement factor exponent takes the form !2nꢄ [hꢄa*ꢄ ;ꢁꢁ#2#2hka*b* ;ꢁꢄ].
Ni(ND ) (NO ) (13) the staggered conformation is intensity. The "t was performed using three (¹4110 K)
ꢃ ꢇ ꢃ ꢄ
observed. The crystal structure of the low-temperature and two Gaussian peaks (¹5120) assigned to (451)#(541),
phases of this compound are unknown.
(235)#(325), impurity and (235)#(325) and impurity, re-
spectively. The sum of re#ection intensities (235)#(325) is
plotted as a function of temperature in Fig. 11, con"rming
a continuous decay of re#ection intensities up to the phase
Temperature Dependence of Ordering in Mg(ND ) Cl
3
2
2
In order to characterize the order}disorder transition in transition at about 135 K. Attempts to "t the temperature
Mg(ND ) Cl neutron di!raction data were recorded dependence of the re#ection intensities according to a criti-
ꢃ ꢄ
ꢄ
between 50 and 150 K with j"1.8857 A
s
. As expected, the cal behavior su!er from the apparent deviation from
superstructure re#ections become weaker with increasing a simple behavior observed around 100 K. Therefore, no
temperatures, "nally disappearing between 130 and 140 K, details are given.
as shown in Fig. 11.
The continuous order}disorder phase transition Ibmm
Comparing the distributions of D atoms observed at P Cmmm is compatible with Landau theory as the order of
and 295 K and the models used for their description space groups changes by a factor of two (24). For
8
reveals that the D density must be redistributed in a rela-
tively complicated way. Furthermore, taking into account
the lower range of d spacings available with the wavelength
of 1.8857 A
taken with 1.4930 A
s
compared to the measurements at 8 and 295 K
neutrons we did not try to evaluate the
s
D densities in detail. Instead, we restricted our analysis on
an evaluation of superstructure re#ection intensities. This
was performed on the overlapping (due to pseudotetragonal
cell dimensions) pair of strong superstructure re#ections
(
235) and (325). In order to determine the sum of their
intensities as a function of temperature the angular range of
3.53(2h(99.53 was chosen to be "tted. This is shown for
the 1.4930 A data in Fig. 3c. The re#ections additional to
235)#(325) in this range can be assigned to an additional
9
s
(
pair or superstructure re#ections (451)#(541) of lower
intensity than (235)#(325) as well as to a weak re#ection
which must be assigned to be originated by an impurity. The
latter is also present above the phase transition temperature.
In this range there is additionally a fundamental re#ection
FIG. 11. Sum of intensities of the re#ections (235) and (325) as a func-
tion of temperature illustrating the continuous phase transition Ibmm
P Cmmm in Mg(ND ) Cl . The intensities were obtained by peak "tting
ꢃ ꢄ
ꢄ
(006) ((003) with respect to the HT phase) with negligible using a Gaussian peak shape.

4
98
LEINEWEBER ET AL.
TABLE 6
Distances and Angles in Mg(ND ) Cl and Mg(ND ) Br
3
2
2
3 2
2
Mg(NH ) Cl
295 K
Mg(NH ) Cl
8 K
Mg(NH ) Br
270 K
Mg(NH ) Br
1.5 K
ꢃ ꢄ
ꢄ
ꢃ ꢄ
ꢄ
ꢃ ꢄ
ꢄ
ꢃ ꢄ
ꢄ
Distances (A)
s
Mg}
4;X
2.567(2)
2.125(3)
0.961(5)
2;X
2.564(5)
2.545(5)
2.137(2)
4;X
2;N
2.754(2)
2.108(3)
2;X
2;X
2.723(5)
2.724(5)
2.092(8)
2.132(8)
0.972(8)
0.981(5)
0.951(7)
0.990(5)
2
;X
2
;N
2;N
1;N(1)
1
;N(2)
N/N(1)}
N(2)}
D}
4;D
1;D(a)
;D(b)
0.976(5)
1.008(3)
2;D
2;Dꢀ
0.915(6)
0.974(7)
1;D(1a)
2;D(1b)
1;D(2a)
2
2
;D(2b)
2;D(1a)2Br
1;D2Cl
2;D(a)2Cl
1;D2Br
1;Dꢀ2Br
2
.649(4)
2.805(2)
2.839(6)
2.752(8)
2.982(6)
2.756(5)
1
;D(b)2Cl
1;D(1b)2Br
2
.570(4)
2
;D(2a)2Br
;D(2b)2Br
2
.966(6)
.745(5)
1
2
Angles (3)
D}N}D
2;79.9(4)?
2;105.2(3)
1;103.4(4)
1;79(1)?
1;78(1)?
2;80.3(4)?
N(1):
1;101.3(6)
2;104.1(6)
N(2):
2
;78.9(4)?
2
;129.2(7)?
2
;129.6(6)?
1
2
;98.4(6)
;103.7
Mg}N}D
N}D2X
4;115.4(3)
1;117.0(4)
2;114.4(5)
2;116.4(6)
N(1):
2
;112.5(3)
1;117.8(6)
2
;113.8(5)
N(2):
1
2
;119.2(7)
;114.6(4)
D(1a):
D:
D(a):
D:
1
;167.1(3)
2;137.7(2)
1;165.4(6)
Dꢀ:
2;132(1)
D(1b):
D(b):
1
;153.6(2)
1;163.3(3)
1;155(1)
D(2a):
2
;135(1)
D(2b):
1
;156(2)
176(1)
N}Mg}N
Bridging
X}Mg}X
180
86.0(2)
176.3(4)
86.50(6)
180
87.3(1)
86.84(6)
?
These angles are not bonding angles but result from the description of the mean distribution within the disordered structure by split sites of D.
Mg(ND ) Br the phase transition Pnam P Pbam is tional parameters will not be discussed for reasons men-
ꢃ ꢄ
ꢄ
expected to occur in a similar fashion. Probably, the tioned above. Lattice parameters and unit cell volumes,
transition temperature of the bromide is lower than that of normalized to the values obtained at 8 K, are plotted in
the chloride, which is also observed for many hexaammines Fig. 12. The unit cell volume shows no anomaly at the phase
(11).
transition temperature. Furthermore, the cell parameters
Cell parameters for Mg(ND ) Cl as a function of tem- show no discontinuity at that point. A continuous phase
ꢃ ꢄ
ꢄ
perature were determined by Rietveld re"nement. Structural transition demands this. However, the anisotropy of ther-
models used for data at ¹4100 K were those of the low- mal expansion changes at the phase transition. Below the
temperature phase, at higher temperatures, for simplicity, temperature of the phase transition, c increases strongly
those of the high-temperature phase. Thermal and posi- compared to the other axes. This is explained by increasing

UNIAXIAL ORIENTATIONAL ORDER}DISORDER TRANSITIONS
499
ACKNOWLEDGMENTS
D1A measurements were performed during CRG-funded beam time.
A.L. acknowledges "nancial support during the HRPT measurement by
the Laboratory for Neutron Scattering of ETHZ and PSI. Furthermore,
"nancial support by the &&Bundesministerium f uK r Bildung, Wissenschaft,
Forschung und Technologie'' (JA5DOR) and the &&Fonds der Chemischen
Industrie'' is gratefully acknowledged. We thank Petra Schiebel (Institut
f uK r Kristallographie, Universit aK t TuK bingen) for discussion.
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2
. A. Leineweber, M. W. Friedriszik, and H. Jacobs, J. Solid State Chem.
1
47, 229 (1999).
. C. H. MacGillavry and J. M. Bijvoet, Z. Kristallogr. 94, 249 (1936).
ttger,
FIG. 12. Cell parameters and unit cell volumes for Mg(ND ) Cl ,
ꢃ ꢄ
normalized to the values at 8 K. < and c refer to the translation vector of
the high-temperature phase, which is doubled in the low-temperature
ꢄ
3. A. Leineweber, M. W. Friedriszik, H. Jacobs, R. Essmann, G. Bo
K
ꢈ
ꢈ
F. Fauth, and P. Fischer, Z. Kristallogr. Suppl. 16, 46 (1999).
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5
. A. Leineweber and H. Jacobs, J. Solids State Chem. 152, 381 (2000).
. R. W. G. Wycko!, &&Crystal Structures,'' Vol. 3, p. 783. Interscience,
New York, 1965.
phase. The arrow indicates the phase transition temperature.
6
7
8
. I. Olvson, Acta Crystallogr. 18, 889 (1965).
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. R. E{mann, G. Kreiner, A. Niemann, D. Rechenbach, A. Schmieding,
T. Sichla, U. Zachwieja, and H. Jacobs, Z. Anorg. Allg. Chem. 622, 1161
repulsive interactions between ND groups with increasing
disorder along the chains.
ꢃ
(1996).
9
. A. Hoser, W. Prandl, P. Schiebel, and G. Heger, Z. Phys. B Condensed
Matter 81, 259 (1990).
CONCLUSION
1
0. J. M. Janik, J. A. Janik, A. Migda"-Mikuli, E. Mikuli, and K. Otnes,
Physica B 168, 45 (1991).
For Mg(ND ) Cl and Mg(ND ) Br uniaxial orienta-
ꢃ ꢄ
ꢄ
ꢃ ꢄ
ꢄ
tional order}disorder phase transitions were characterized 11. L. Asch, G. K. Senoy, J. M. Friedt, J. P. Adlo!, and R. Kleinberger,
J. Chem. Phys. 62, 2335 (1975).
by means of neutron di!raction. In the low-temperature
1
2. P. Schiebel, Institut fu
communication.
3. A. F. Andresen, H. Fjellvag, J. A. Janik, J. Mayer, J. S
K r Kristallographie, Universit aK t TuK bingen, private
modi"cations the D atoms are ordered in contrast to the
situation in the high-temperature phase stable at ambient
temperatures.
1
D
ciensi nH ki,
s
J. M. Janik, A. Migda"-Mikuli, E. Mikuli, M. Rachwalaska, and T.
Stanek, Physica B 138, 295 (1986).
An interesting comparison is that between ammines and
hydrates. Mg(H O) Cl (25) is isotypic to the corresponding
1
1
4. U. Zachwieja, Ph.D. thesis, Universit aK t Dortmund, 1991.
5. H. Jacobs, R. Niewa, T. Sichla, A. Tenten, and U. Zachwieja, J. Alloys
Compd. 246, 91 (1997).
ꢄ
ꢄ ꢄ
Mn, Fe, and Co compounds (26, 27) and contains octahed-
ral chains ꢁ[Mg(H O) X ], which are arranged relative to
1
1
6. H. Jacobs and D. Schmidt, Curr. ¹op. Mater. Sci. 8, 379 (1982).
7. B. N oK cker, Ph.D. thesis, Universit aK t Dortmund, 1991.
ꢂ
ꢄ
ꢄ ꢅꢆꢄ
ꢃ ꢄ
each other as in Mg(NH ) Cl . The symmetry is, however,
ꢄ
monoclinic, C2/m, caused by uniaxial rotational ordering of 18. P. Fischer, G. Frey, M. Koch, M. Ko
K
nnecke, V. Pomjakushin,
J. Schefer, R. Thut, N. Schlumpf, R. BuK rge, U. Greuter, S. Bondt, and
the H O molecules which is present at ambient temper-
ꢄ
E. Berruyer, Physica B 276+278, 146 (2000).
atures. The H O molecules can adapt well to the local
ꢄ
19. A. W. Hewat and I. Bailey, Nucl. Instrum. Methods 137, 463
environment of X atoms. Both H atoms point directly to an
(1976).
X atom. Therefore, no frustration of the interactions
H 2 X occurs, in contrast to the ammines. Hydrates crys-
tallizing similarly to the Mg(NH ) Br type structure are
2
2
0. A. C. Larson and R. B. von Dreele, General Structure Determination
System (GSAS). Los Alamos National Laboratory, Los Alamos, NM,
1
995.
ꢃ ꢄ
ꢄ
1. P. Schiebel, A. Hoser, W. Prandl, G. Heger, W. Paulus, and P.
Schweiss, J. Phys. Condens. Matter 6, 10989 (1994).
2. A. R. Bates and K. W. H. Stevens, J. Phys. C 2, 1573 (1969).
3. Th. Hahn, &&International Tables for Crystallography,'' Vol. A. Kluwer
Academic Publishers, Dordrecht, 1995.
4. J. C. Tol eH dano and P. ToleH dano, &&The Landau Theory of Phase
Transitions.'' World Scienti"c, Singapore, 1987.
5. A. Leineweber, R. Niewa, and H. Jacobs, unpublished.
6. B. Morosin and E. J. Graeber, Acta Crystallogr. 16, 1176 (1963).
7. B. Morosin and E. J. Graeber, J. Chem. Phys. 42, 898 (1965).
not known. Mn(H O) Br and Fe(H O) Br are isotypic to
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
the corresponding chlorides (28).
2
2
The oberserved changes of the long-range order in the
ammines presented in this paper which results from the
evaluation of elastic neutron di!raction considering Bragg
re#ections are most probably accompanied by changes of
2
2
2
2
the rotational dynamics of the ND groups. However, spec-
ꢃ
troscopic methods are needed to get a deeper insight in the
dynamics in these compounds.
28. B. Morosin, J. Chem. Phys. 47, 417 (1967).