Revolución digital Enfoque –

(i) Bulk reactions

All reactions o f Group 3-5 transition metal halides and rare earth halides with lithium nitride yielded crystalline powders with crystallite size o f the order o f 250-1200 Â, as calculated from XRD line broadening using the Scherrer equation using Formula 2.1 below:

L = kA,/cos0'v/(B^ - b^) Formula 2.1

wiiere, L = crystallite size, k = constant (0.89 < k < 1.1 ), X, = X-ray wavelength, 0 = diffraction angle, B = halfwidth of reflection for sample, b = halfwidth for standard peak ([101] of zinc)

Crude and triturated products showed that purification using solvent washing completely removed the co-formed salt (figure 2.1). (continued on page 56)

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Figure 2.1 XRD patterns o f crude (a) and triturated (b) product from bulk reaction o f

yttrium trichloride with lithium nitride. Bottom stick patterns are for yttrium nitride (c), lithium chloride hydrate. LiCl.HjO and lithium chloride (dashed lines) (d).

T able 2.1 Selected reactions o f metal halides with lithium nitride or sodium azide initiated with flam e (fl), filament (fil) or performed in hulk under vacuum (bkv) or nitrogen pressure (bkp). * Literature lattice parameters were taken from reference [152] and [159J.

Reaction Products (after

trituration)* +/-0.001

aut! k

Sca3 + Li3N(bkv) ScN 4.488 4.45

S cC b + L13N (fil) No reaction

SCCI3 + NaN3 (bkv) ScN 4.491 4.45

TICl3 + Li3N(bkv) TiN o, 4.238 TiNo 8 4 = 4.238

TiQ3 + U3N(fil) TiNo69 4.231 TiNo 6 9 = 4.231

T:Cl3 + NaN3 (bkv) TiNo, 4.236 TiNo 8 1 = 4 .2 3 6

TiCl3 + Li3N/NaN3 (bkv) TiN o. 4.240 TiNo 90 = 4.240

ZrCl4 + Li3N(fll) ZrN_1.00 4.577 ZrN, 0 0 = 4.577

ZrC U + U 3N (bkv) ZrNo« 4.570 ZrNo,95 = 4.570

HfCU + L13N (bkv) HfNo, 4.520 HfNo 8 3 = 4.520

H fC lt + U 3N /NaN3 (bkv) HfN1.09 4.516 HfN, 09 = 4.516

\ d s + LI3N (bkv) VNo89 4.111 V No8 9 = 4 . 1 1 1

V a3 + Li3N (f il) VNo, 4.098 VNo 88 = 4.098

VCl3 + NaN3 (bkv) VNo, 4.112 V N o 9 3 = 4.112 NbCIs + L13N (bkv) NbN,06 4.399 NbN, 06 = 4.399

TaCIs + L13N (bkv) TaN [TajN] (TaN) 4.294 4.33

CrCIj + LI3N (bkv) Cr2N(hex)

[CrN] (CrzN) a = 4.776, c= 4.454 a = 4.805, c = 4.479 CrCl3 + U 3N ( bkv) Cr2N(hex) a = 4.772, c = 4.479 a= 4.805, c - 4.479 CrCl3 + L iJ t (bkv) CrN, Cr (CrN) 4.148 4.150 M 0 C I 3 + L Î 3 N ( b k v ) Mo L»Cl3 + U 3N (bkv) LaN 5.295 5.30

LaCl3 + NaN3 (bkv) LaN 5.277 5.30

E uC b + U 3N (bkv) EuN 4.99 5.01

E u C b + NaN3 (bkp) EuN 5.01 5.01

AICI3 + U 3N (bk v)' AIN(hex), AI (AIN) a - 3 .1 0 9 , c - 4.972 a = 3.110, c = 4.975 AICI3 + Li3N/NaN3 (bkv)^ AI AICI3 + L13N (bkp)" AIN(hex) a = 3.110, c = 4.979 a = 3.110, c = 4.975

LIAlCl4 + Ll3N(bkv)* AIN, A1 (AIN) 7.927 a = 7.913

LlAICl4 + Li3N/NaN3 (bkv)*

LiAICl4 + L i3N (bkp)*

AI

AIN a= 7.935 a = 7.913

InCl3 + U 3N (bkv) InN(hex), (In) (InN) a= 3.49,

c= 5.61

a = 3.54, c= 5.70 *AI1 the reaction mixtures were always heated to 550 °C within 30 minutes and then cooled down ready for trituration. Most of these reactions initiated at temperatures below 500 °C. In the reactions performed under vacuum it is has been assumed that no nitrogen is lost from the reaction mixture. In the reactions where lithium nitride and sodium azide were used together the standard molar ratio applied was 3 : 1 (Li^N : NaN^). "all nitride products cubic unless stated.

^reaction required annealing for 24-36 hours. [ ] - minor phases present.

T he p roducts from the reactions o f transition m etal halides and various nitriding reagents w ere highly crystalline considering the reaction tim escale. This is due to tw o factors, w hich participate in the SSM reactions:

(i) H igh reaction enthalpies as dem onstrated by the exam ples in e q u a t io n s

2.11-2.16 in section 2.5. The theoretical m axim um reaction tem peratures as calculated assum ing adiabatic processes are expected to be close to the tem peratures reached in practice due to the short tim escale o f the SSM processes.

(ii) F orm ation o f m olten salts w hich may catalyse crystallisation o f SSM p r o d u c t s . L o w e r m elting point salts will rem ain m olten for longer allow ing m ore tim e for recrystallization o f the product.

Table 2.2 Selected reactions o f metal halides with Group 2 nitrides performed usins flame

(fl) or filament (fil) initiation or as a bulk reaction under vacuum (bk).* Literature lattice parameters were taken from references fl5 2 1 and [159].

R e a c t i o n P roducts a fte r tritu ra tio n and

annealing^

a o b . / A + /- 0 .0 0 5 a i i t l k

S cC b + MfoNz (bkv) ScN 4.493 4.45 ScC b + C ajN i (fil) No reaction

Y C b + M foN i ( bkv) YN 4.862 4.89 T f tl, tC a iN z (bkv) YN 4.897 4.89 TiCIa + MgjNz (bk) TiNo.52 4.224 TiNo 52 - 4.224

VCl3 + C ajN 2 (b k ) VNo.92 4.117 VNo,9 2 -4 .1 1 7 v a 3 + Ca3N2(fU) VNo.89, V2N (VN) 4.110 VNo89 = 4.110 V a 3 + M foN 2(bk) VNo.97 4.121 VN0 9 7 = 4.121

v a 3 + M gjN 2(fil) No reaction

T a a s + Ca3N2(fiI) TaN 4.31 4.33 T a C Is+ Ca3N2 (II) TaN 4.28 4.33 T a C Is+ M gd^ 2 (fil) No reaction

T a C Is+ M gjN2 (fl) TaN, TajN’’

CrCl2 + M foN 2(bk) CrN [CrjNChex)] (CrN) 4.151 4.150 CrCl3 + M foN 2(bk) CrN, Cr2N(hex) (CrN) 4.150 4.150 CrCl2 + Ca3N 2(bk) CrN, CrjNChex) (CrN) 4.150 4.150 CrCl3 + Ca3N 2(bk) CrN, Cr2N(hex) (CrN) 4.149 4.150 M0CI3 + M gjN2 (bk) Mo M0CI5 + MgsN2 (bk) Mo

AICl3 + Ca3N2(fI) AIN(hex) [Al] a = 3.114, c = 4.984 a = 3.110, 6 = 4.975 L aC b + Mg3N2 (bkv) No reaction

L a C b + Ca3N2 (bkv) No reaction

*A1I bulk reaction mixtures were brought to 550 °C and cooled down for trituration. Most o f the bulk reactions initiated below 400 °C. Products from the reactions initiated with flame or filament were not annealed. [] - m inor phases present, ‘’sample poorly crystalline and the lattice parameter was not calculated, *all nitride products cubic unless stated.

The lattice parameters calculated for the products from the metathesis reactions were correlated with the level o f nitridation. Goldschmidt determined that the lattice parameter is sensitive to the degree o f nitridation where a decrease in the amount o f nitrogen in the product corresponds to a decrease in the a-

parameter in cubic nitrides. These can be correlated with the Vegards Law and shows a linear relationship.^^*

Reactions o f Group 6-9 transition metal halides with alkaline or alkaline earth metal nitrides yielded only pure metal phases with nitrogen gas being released. This is due to the fact that the reaction temperatures generated are above the nitride decomposition point. The reactions o f chromium halides were the only exception yielding mixed or single nitride phases and often chromium metal (Tables 2.1 and 2.2). Group 3-5 nitrides have much higher decomposition temperatures than group 6-9 nitrides. This has been referred to as the ‘Chromium enigma’.

(ii) Propagation reactions

Reactions initiated using a filament form solids o f smaller particle size than those initiated using flame. Figure 2.2 shows the products obtained from the reaction o f zirconium tetrachloride and lithium nitride in bulk and filament initiated processes. The zirconium nitride formed in the bulk reaction was more crystalline. The crystallite sizes o f the solids prepared using propagation reactions, calculated using the Scherrer equation, were o f the order o f 50-600 A.

2.3.2 SEM/EDXA analysis

SEM measurements o f the ‘as prepared’ solids from all modes o f initiation showed large agglomerates with smooth morphologies. Triturated samples were typified by having smaller size agglomerates with sharper angles and faces (figure 2.3). Some crystals within agglomerates were o f significantly large dimension. An example o f this is the reaction o f transition metal halides with lithium nitride and sodium azide (ratio 3 :1 ) . Figure 2.4 shows large crystallites on the face o f an agglomerate o f the product from the reaction o f titanium trichloride with lithium nitride and sodium azide. Transition metal nitrides

(a) (b)

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Figure 2.2 Triturated products from the reactions o f zirconium tetrachloride with lithium

nitride perform ed inside ampoule (a) or initiated with a filam ent (b). Bottom stick pattern is for rocksalt zirconium nitride, ZrN.

obtained from the reactions of transition metal halides with Group 1 and 2 nitrides or sodium azide showed narrow particle size distributions within the samples.

EDXA analysis o f pretriturated products showed the presence o f transition metal (or rare earth metal). Group 1 or 2 metal and halogen (nitrogen was below the threshold o f the EDXA system used). However it was found that most o f the surface of the particles was layered with the co-product salt. Triturated product analysis showed only metal present.

The morphology o f products prepared at different reaction temperatures showed little variation across the whole transition metal block. The most visible distinction observed in the numerous products analysed was associated with the mode o f initiation. Products prepared using a filament for initiation were less pure and had smaller aggregates than those isolated from bulk reactions. A non- uniform product particle size was frequent. Higher quality products were obtained from bulk reactions. An increase in the reaction scale assisted an improvement in product quality.

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Figure 2.3 SEM micrographs o f triturated products from the reactions o f titanium

trichloride with lithium nitride and sodium azide (a), vanadium trichloride with lithium nitride (b), titanium trichloride with lithium nitride (c), hafnium tetrachloride with lithium nitride and sodium azide (d), and surface o f particle shown in (d), (e).

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Xi üÔK

0 0 0 0 4 0 25KV

Figure 2.4 Crystallites on the face o f an asslom erate o f titanium nitride prepared by the

bulk reaction o f titanium trichloride with a lithium nitride/sodium azide mix (molar ratio 3 : I respectively).

On selected samples linescan measurements using an instrument with a thin-window detector were observed. This system allowed semi-quantification of nitrogen. Figure 2.5 shows the results obtained from linescan tests performed on hafnium nitride prepared in a bulk reaction of hafnium tetrachloride and lithium nitride under nitrogen pressure. Figure 2.5a shows the surface of the sample as seen in the EDXA system, monitored with a line, across which, the measurement was performed. Figure 2.5b shows plots of linescan lengths against the intensities for nitrogen and hafriium in arbitrary counts. The patterns obtained show clearly a match between hafriium and nitrogen indicating their relationship as part of one compound. The presence of oxygen has also been tested and the results showed that the sample surface is somewhat contaminated with oxygen. This is expected with this class of compounds since they readily form an oxide layer on their surface. Similar results have been obtained for the other transition metal nitrides analysed.