1 A Layered Spin 12 polymorph of titanium triiodide Danrui Nia Ranuri S. Dissanayaka Mudiyanselage b Xianghan Xua Junsik Mun d Yimei Zhu d

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A Layered Spin 1/2 polymorph of titanium triiodide

Danrui Ni,a Ranuri S. Dissanayaka Mudiyanselage,b Xianghan Xu,a Junsik Mun,d Yimei Zhu,d

Weiwei Xie,c and Robert J. Cavaa

aDepartment of Chemistry, Princeton University, Princeton, NJ 08544, USA

bDepartment of Chemistry and Chemical Biology, Rutgers University, Piscataway, NJ, 08854,

USA

cDepartment of Chemistry, Michigan State University, 578 S Shaw Lane, East Lansing, MI 48824,

USA

dCondensed Matter Physics and Materials Science Department, Brookhaven National

Laboratory, Upton, New York 11973, USA

Abstract

A previously unreported layered spin ½ triangular lattice polymorph of TiI3 is described,

synthesized under 6 GPa of applied pressure at 900 C, but stable at atmospheric pressure. This

air-sensitive material has a CdI2-type layered structure (P-3m1 (#164), a = 4.012 Å and c = 6.641

Å at 120 K, Z = 1 of Ti0.667I2) with an in-plane triangular lattice, related to that of TiI4 (Ti0.5I2).

Although the TiI3 formula is consistent with expectations for a layered honeycomb lattice of

spin ½ Ti(III), there is disorder in the crystal structure. Magnetic susceptibility and heat capacity

measurements suggest that the material undergoes several low temperature phase transitions.

Keywords: Titanium triiodide; High pressure synthesis; Spin ½; Layered structure;

Polymorphism.

Introduction

Metal halides, especially transition metal halides, have attracted much research interest in

recent years. Many members in this large family including 3D perovskites [1], 2D layers

(triangular [2,3], Kagomé [4], or honeycomb [5–7]), and 1D chains [8,9] have been reported.

Polymorphic behavior is also known [10–12]. The materials can display unexpected physical

and/or photochemical properties, and thus promising performance in photovoltaic, electric,

and quantum material applications [13–16]. It is therefore of great interest to characterize new

phases of transition metal halides and study their physical and chemical properties. Here we

describe the disordered layered triangular lattice spin ½ material TiI3 synthesized at 6 GPa, its

crystal structure is very different from that of the ambient pressure synthesized phase, which is

made from titanium iodide chains.

Among the reported transition metal trihalides, TiCl3, with Ti3+ in d1 configuration, was found to

have both layered and chain-structure polymorphs as early as 1961 [10]. The triiodide variant,

TiI3, however, has only been reported to display a 1D chain structure. Iodides can display

properties that are much different from their chloride cousins and are therefore worth

finding [17], and a potential two-dimensional phase consisting of spin ½ metals in a layered

lattice with iodine atoms is theoretically expected to display interesting electronic and magnetic

properties [18]. The layered polymorph of spin 1/2 TiI3 has not been reported experimentally

until now. We report it here but find it to display a disordered Ti sublattice when synthesized

under our conditions, which may be due to either stacking faults or in-plane Ti-vacancy

disorder, the latter being the most likely.

As a starting point for understanding the synthesis of the current material, we note that

pressure-induced polymorphism has been reported in the well-studied ruthenium trihalides.

While RuCl3 undergoes a phase transition from its chain-structure β-phase to a honeycomb-

layered α-phase with increasing temperature, for its sister compounds RuBr3 and RuI3 only

chain structures have been reported at ambient pressure; with the help of applied pressure,

however, the honeycomb layer polymorphs of RuBr3 and RuI3 have been experimentally

synthesized [17,19,20]. In that vein, here we employ modest temperatures and pressures to

synthesize a layered polymorph of spin ½ TiI3. Layered TiI3 is air-sensitive (as is the ambient

pressure polymorph) and is refined to have a P-3m1 CdI2-type structure with d1 Ti(III) on

average partially occupying 2/3 of the Cd-type sites. This spin ½ material is structurally related

to the d0 compound TiI4 (Ti0.5I2) whose structure consists of zig-zag chains of edge-shared TiI6

octahedra within a layered iodine array. Possible explanations for the partial occupancy of

titanium in the triangular layered lattice are discussed, and magnetic susceptibility and heat

capacity measurements are carried out, suggesting the presence of phase transitions in TiI3 in

the lower temperature range.

Experimental

The ambient-pressure phase of TiI3, used as a starting material, was synthesized using

elemental titanium powder (Alfa Aesar, 99.9%) and iodine (Sigma Aldrich, 99.99%). It was

annealed in vacuum in sealed quartz tubes for three days, with multiple annealing

temperatures between 300 – 700 C tested to increase the yield. Some impurities were

produced during the annealing, including TiI4, and TiI2. A low temperature vapor transport

method was used for purification before the high-pressure synthesis. With the hot zone kept at

350 C, impurities such as TiI4 are transferred to the cold end, leaving pure TiI3 in the region

with higher temperature. The resulting black-colored, 1D-chain structure TiI3 material was used

as starting material for the high-pressure synthesis. TiI4 was also employed for high pressure

synthesis experiments, as described below.

The ground powder of the starting material was loaded into a boron nitride crucible and then

inserted into a pyrophyllite cube assembly. The samples were pressed to 6 GPa using a cubic

multi-anvil system (Rockland Research Corporation) and annealed at different temperatures

(700 – 1000 C), with the temperature measured by an internal thermocouple. The samples

were then quench-cooled to ambient temperature before decompression. Annealing at 900 °C

for 1 hour gave the best results. The products obtained displayed a black color. Small single

crystals were picked up in the post-reaction samples and were used for single crystal X-ray

diffraction (SCXRD) characterization of the crystal structure. All the titanium iodide compounds

prepared require handling in an air-free atmosphere. Thus the PXRD patterns used for

characterization were collected using a Rigaku Miniflex II diffractometer located inside a

nitrogen-filled glove box. Cu Kα radiation (λ= 1.5406 Å) was employed, and Le Bail fitting of the

acquired patterns, when performed, was conducted via the TOPAS software.

SCXRD data of TiI3 crystals was collected at 120 K on a Bruker D8 Quest Eco using graphite-

monochromated Mo Kα radiation (λ = 0.71073 Å). A liquid nitrogen stream was used to prevent

samples from decomposing. The frames were integrated using the SAINT program within the

APEX III 2017.3-0 operating system. The structure was determined using direct methods and

difference Fourier synthesis (SHELXTL version 6.14) [21]. The P-3m1 (#164) space group was

suggested by XPREP. Other potential space groups (P-31c, P63mc, P63/mmc and P6/mmm)

were also tested but structures in those space groups did not lead to satisfactory or better

refinements, with either higher R/wR2 or higher Rint values. The electron microscopy

measurements were conducted on ground powder of TiI3 but the beam-sensitivity of the

sample significantly limited its characterization by this method.

The extreme air sensitivity of the high-pressure-synthesized compound makes getting

quantitative magnetic susceptibility and heat capacity data difficult, but magnetization and heat

capacity measurements were conducted in any case, with an effort made to minimize contact

of the material with air. The experiments were carried out using a Quantum Design PPMS

(Dynacool), equipped with a vibrating sample magnetometer (VSM) option. For these and all

other cases, transfers were performed very rapidly to prevent sample decomposition. Magnetic

susceptibility was defined as M/H, and the temperature-dependent magnetization (M) was

measured in an applied magnetic field (H) of 1000 Oe.

Results and Discussion

1D chain TiI3 (Pmnm [22]) and cubic TiI4 (Pa-3 [23]) were synthesized and purified at ambient

pressure, and were used as starting materials for the high-pressure synthesis (Figures S1 and S2

show their ambient temperature PXRD patterns). Optimized synthesis conditions for the high

pressure TiI3 phase were found to be 900 C and 6 GPa for 1 hour. The high-pressure TiI3 phase

(HP-TiI3) obtained crystallizes in a layered structure in the P-3m1 space group (#164) with a =

4.012 Å and c = 6.641 Å at 120 K (Figure 1). The crystallographic information is presented in

Tables 1-3 and Table S1. During the refinement, pseudo symmetry elements (i.e. a 63 screw and

a c glide) were suggested by Checkcif but they were found to not be present. The P-3m1

structure yields the best refinement. The hk0, hk1 and 0kl reciprocal planes of HP-TiI3 are

shown in Figure 2A.

HP-TiI3 crystallizes in a CdI2-type layered structure with a triangular lattice, where the Ti on

average randomly occupies the Cd site, leading to an occupancy ratio of 0.667Ti:2I when

considered on an MX2 structural basis. Compared to the low-temperature structure of ambient

pressure chained TiI3, which was reported at approximately the same temperature [22], the TiI6

octahedra in the HP material are less distorted. The Ti-I bond length is about 2.82 Å, and the TiI6

polyhedron is closer to that of a regular octahedron. When compared to high-temperature

chain-structure AP-TiI3 (P63/mcm space group with more symmetric Ti-coordination), the

bonding distance is slightly longer in HP-TiI3 (Table 3). The 300 K PXRD pattern of HP-TiI3 is

consistent with the SCXRD results (confirmed by Le Bail fitting in Figure S3), yielding an a of

4.0277(2) Å and a c of 6.6828(2) Å at 300 K for the P-3m1 unit cell. A small amount of ambient

pressure TiI3 is found in the HP-TiI3 PXRD pattern - less than 9% based on a rough Rietveld

refinement.

After an analogous high-pressure high-temperature treatment, TiI4 shows a PXRD pattern that

indicates that it also has a trigonal crystal structure. Some impurities remain present, including

monoclinic TiI4, whose crystal structure [24] is closely related to that of trigonal HP-TiI3. 1D

chain TiI3 and titanium metal are also present (Figure S4). The cubic TiI4 polymorph, which was

the starting material for the HP synthesis, is reported to be metastable at 300 K, slowly

transforming to monoclinic TiI4 at ambient pressure [23], so it is not surprising that this phase

was not observed in the post-reaction PXRD pattern. By viewing the PXRD patterns of the high

pressure synthesized TiI3 and TiI4 together (Figure 2B), small shifts of the peaks are revealed.

The similar diffraction patterns suggest that HP-TiI4 and HP-TiI3 are essentially isostructural, but

that due to the different Ti:I ratios in the starting materials, HP-TiI4 has a different occupancy of

Ti on the 1a Wyckoff site in the crystal structure. The Le Bail fitting of the 300 K PXRD pattern

for HP-TiI4 yields a = 4.0178(2) Å and c = 6.6299(3) Å, with these smaller values being consistent

with the expected lower radius of d0 Ti4+ compared to that of d1 Ti3+. This structure type has

also been observed for TeI4 [25], as one of its polymorphs was found to adopt a CdI2-type

structure (Te0.5I2) with a random distribution of Te and vacancies over the potential metal

positions in the layered triangular lattice. Another comparable example is OsxCl3 [26,27], which

will be discussed in a later section. Based on the similarity of the two PXRD patterns, the HP

titanium iodide series in the composition range studied may be a solid solution with the same

P-3m1 layered structure but with different Ti content on the 1a site, i.e. resulting in an average

formula of TixI2, with x varying at least between 0.5 and 0.667. As some reports suggest that TiI2

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1ALayeredSpin1/2polymorphoftitaniumtriiodideDanruiNi,aRanuriS.DissanayakaMudiyanselage,bXianghanXu,aJunsikMun,dYimeiZhu,dWeiweiXie,candRobertJ.CavaaaDepartmentofChemistry,PrincetonUniversity,Princeton,NJ08544,USAbDepartmentofChemistryandChemicalBiology,RutgersUniversity,Piscataway,NJ,08854,USAcDepar...

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1 A Layered Spin 12 polymorph of titanium triiodide Danrui Nia Ranuri S. Dissanayaka Mudiyanselage b Xianghan Xua Junsik Mun d Yimei Zhu d

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