Search results for "transition"
showing 10 items of 3988 documents
The Phase Behaviour of Amphiphilic Monolayers
1991
Amphiphilic monolayers display a wealth of condensed phases. It is only recently that these have been widely recognised as being distinct, but they are in fact quite analogous to the well-known lamellar gel phases of bulk lyotropic and thermotropic mesogens. An understanding of these phases has proved to be important for controlling the defect structure of the monolayers used in the fabrication of Langmuir-Blodgett films. This understanding is being achieved by developments in a number of techniques, including X-ray diffraction and isotherm miscibility. The phase diagrams of the materials studied so far are all very closely related, lending new credibility to the principle of corresponding …
Structural phase transition to disorder low-temperature phase in [Fe(ptz)6](BF4)2 spin-crossover compounds.
2012
In the spin-crossover compound [Fe(ptz)6](BF4)2 (where ptz=1-n-propyltetrazole) six different phases are observed. When a single crystal is slowly cooled from high temperatures to those below 125 K, the reflections broaden into diffuse maxima and split into two maxima along the c* direction [Kusz, Gütlich & Spiering (2004). Top. Curr. Chem. 234, 129–153]. As both maxima are broad along the c* direction, the short-range order exists only along the c direction and in the ab plane the structure remains long-range ordered. In this disordered phase additional satellite reflections appear. Upon heating above 135 K, the diffuse maxima return to their previous shape and this process is complete…
Phase transition in bis(ethyldimethylammonium) pentachloroantimonate(III)
1999
The crystal structure of a new member of the alkylammonium chloroantimonate family, namely bis-(ethyldimethylammonium) pentachloroantimonate(III), [C 2 H 5 NH(CH 3 ) 2 ] 2 [SbCl 5 ], was determined at room temperature. The anionic sublattice consists of isolated distorted square-pyramidal [SbCl 5 ] 2- units and two crystallographically inequivalent disordered ethyldimethylammonium cations connected to anions by NH...Cl hydrogen bonds. The disorder is realised by the presence of two positions for all C, N and H atoms, and is explained as being a result of the overall reorientation of cations along the long axis of the molecule. One second-order phase transition was found at ca 160 K and resu…
Polymorphism and “reverse” spin transition in the spin crossover system [Co(4-terpyridone)2](CF3SO3)2·1H2O
2009
[EN] Compound [Co(4-terpyridone)(2)](CF3SO3)(2)center dot 1H(2)O, where 4-terpyridone is 2,6-bis(2-pyridyl)-4(1H)-pyridone, forms two polymorphs. Polymorph 1 displays a continuous spin conversion in the temperature region 300-120 K while polymorph 2 shows, on cooling, the onset of a continuous high-spin (HS) to low-spin (LS) conversion interrupted by an abrupt "reverse'' spin transition in the temperature region 217-203 K. The formed unstable HS intermediate phase (IP) undergoes a strong cooperative "normal'' spin transition characterised by a hysteresis loop 33 K wide. The structural data give support for a crystallographic phase transition, which takes place concomitantly with the "revers…
Colour centres in LiBaF3 crystals
1998
Abstract The origin of the absorption bands in LiBaF 3 created by X-ray irradiation at RT has been investigated. It is found that three absorption bands at 270, 320 and 430 nm represent different electron transitions within a radiation defect effectively created in LiBaF 3 single crystals. Following the Mollwo–Ivey relation we discuss investigations of optical dichroism, magnetic optical dichroism, as well as the assumptions regarding the accumulation kinetics of these absorption bands, the F centre being the main radiation defect created by X-rays in undoped LiBaF 3 crystals at RT.
X-ray powder diffraction data for some transition metal phosphites and hypophosphites
1994
A set of nine transition metal oxophosphorous compounds, consisting of four hypophosphites and five phosphites has been synthesized and studied using X-ray powder diffraction techniques. The compounds are α-Mn(H2PO2)2·H2O, Co(H2PO2)2 ·0.53H2O, MCl(H2PO2)2·H2O (M = Co2+, Ni2+), M2(HPO3)2·2H2O (M = Co2+, Ni2+), and M11 (HPO3)8(OH)6 (M = Zn2+, Co2+, Ni2+). Unit cells and X-ray powder patterns are reported.
Contribution of X-Ray diffraction to the study of spin transitions in some iron (II) complexes
1991
Abstract A review of recent single crystal investigations under constraint of iron (II) spin crossover complexes is given. The systems covered are Fe L2 (NCS)2 complexes (L = Phen, Btz) and some analogous derivatives. Emphasis is laid on the correlations between structural changes associated with spin transitions and the experimentally determined characteristics of these transitions.
Interplay of Antiferromagnetic Coupling and Spin Crossover in Dinuclear Iron(II) Complexes
2003
This article reports on the study of the interplay between magnetic coupling and spin transition in 2,2′-bipyrimidine (bpym)-bridged iron(II) dinuclear compounds. Coexistence of both phenomena has been observed in [Fe(bpym)(NCS)2]2bpym, [Fe(bpym)(NCSe)2]2bpym and [Fe(bt)(NCS)2]2bpym (bpym = 2,2′-bipyrimidine, bt = 2,2′-bithiazoline) by the action of external physical factors namely pressure or electromagnetic radiation. Competition between magnetic exchange and spin crossover has been studied in [Fe(bpym)(NCS)2]2bpym at 6.3 kbar. LIESST experiments carried out in [Fe(bpym)(NCSe)2]2bpym and [Fe(bt)(NCS)2]2bpym at 4.2 K have shown that is possible to achieve dinuclear molecules with different…
Mixed spin-state [HS-LS] pairs in a dinuclear spin-transition complex: confirmation by variable-temperature 57Fe Mössbauer spectroscopy.
2008
Exquisite sensitivity of Mossbauer spectroscopy for tiny local molecular distortion is demonstrated in [FeII2(pmat)2](BF4)4?DMF: high-spin (HS) iron(II) in [HS-HS] and in [LS-HS] (low-spin–high-spin) pairs is clearly distinguished (see picture) for the first time without the need to apply a magnetic field. This dinuclear complex clearly shows that spin crossover via a [LS-HS] species is promoted by the use of a highly constrained bridging ligand (the bis-terdentate pmat).
X-ray structure study of the light-induced metastable states of the spin-crossover compound [Fe(mtz)6](BF4)2
2001
Iron(II) complexes exhibiting thermal spin-crossover may be converted from the1A1low-spin (LS) state to the5T2high-spin (HS) state by irradiation with green light (light-induced excited spin-state trapping, LIESST) and from the LS to the HS state by irradiation with red light (reverse LIESST). The lifetime of the metastable LIESST states may be sufficiently long to enable an X-ray diffraction study. The lattice parameters of a single crystal of [Fe(mtz)6](BF4)2(mtz = methyltetrazole) (space groupP21/n) were measured between 300 and 10 K. While one Fe lattice site (A) of the crystal changes from the HS to the LS state near 78 K, the other site (B) remains in the LS state. Using the green lig…