BPSC 71st CCE Prelims General Studies · Question 132 of 150

Correct — A, Europium dopped strontium aluminate — the booklet's spelling of europium-doped strontium aluminate. The material is strontium aluminate, SrAl₂O₄, activated with europium in its +2 state and co-doped with dysprosium, written SrAl₂O₄:Eu²⁺,Dy³⁺. It is the pigment behind virtually every glow-in-the-dark emergency exit sign, escape-route marking, watch dial and luminous toy made since the mid-1990s. The europium ion does the emitting; the dysprosium does the remembering. Daylight or room light in the 200–450 nm band lifts electrons out of Eu²⁺, the dysprosium-induced lattice defects trap that charge, and ordinary room-temperature heat then leaks it back to the europium over minutes and hours, each returning electron releasing a green photon peaking at about 520 nm. That stored-and-slowly-released step is persistent luminescence — phosphorescence, not fluorescence — and it is why the pigment keeps working in a blacked-out stairwell. The material has a datable origin: it was developed in 1993, the discovery credited to Yasumitsu Aoki of the Japanese firm Nemoto & Co., and patented by that company in 1994 as US Patent 5,424,006, 'Phosphorescent phosphor'. The motive was safety — the glow paints it replaced used radioactive promethium. Measured against the older copper-activated zinc sulphide it is roughly ten times brighter, glows about ten times longer and costs about ten times more, and it is now the brightest and longest-lasting phosphorescent material available commercially, sold under names such as Super-LumiNova and Seiko's Lumibrite. Crucially it stores nothing but light: strontium aluminate is non-toxic, chemically inert and endlessly rechargeable, unlike the radium dial paints and tritium tubes that preceded it.

• (b)Copper Sulphide dopped zinc oxide — The cleverest distractor on the page, because it sits one swap away from something entirely real. The standard glow-in-the-dark pigment before 1993 was copper-activated zinc SULPHIDE, Cu:ZnS — the greenish material in old glow-in-the-dark stars and toys. This option moves the sulphide onto the copper and hands the host job to zinc oxide, a white pigment used in sunscreen and rubber. Copper sulphide is a dark semiconducting mineral, and the pairing has no persistent-luminescence use at all.
• (c)Barium sulphate dopped copper carbonate — Two genuinely useful compounds put to work in the wrong job. Barium sulphate is the inert white filler of paints and paper and the 'barium meal' swallowed for X-ray contrast; basic copper carbonate is malachite, a green mineral pigment. Neither phosphoresces, and the chemistry rules it out on principle: a persistent phosphor needs a colourless wide-band-gap host, whereas the Cu²⁺ ion is coloured precisely because it absorbs visible light, so it quenches emission rather than producing it.
• (d)Boron oxide dopped copper sulphate — Boron oxide, B₂O₃, is a glass-former and flux — the borosilicate in laboratory glassware, the bead in the borax bead test — while copper sulphate is the blue crystalline salt of Bordeaux mixture and electroplating baths. The same fatal objection applies: copper(II) is intensely coloured and quenches luminescence, and neither compound serves as a phosphor host. Boron does appear in real strontium-aluminate phosphors, but only as a trace additive that lengthens the afterglow — never as a dopant for a copper salt.

A glowing pigment is never a single substance; it is a two-part design — a transparent host crystal plus a trace of a deliberately introduced impurity called the activator. Doping means seeding that impurity ion into the lattice at the level of a fraction of a per cent, the same idea as doping silicon with phosphorus, except that the payoff is optical rather than electrical. In SrAl₂O₄:Eu²⁺,Dy³⁺ the host is strontium aluminate, the activator is europium(II), and dysprosium(III) emits nothing at all: its role is to create lattice defects that act as electron traps. Incoming light excites the europium, the traps capture and hold part of the freed charge, and thermal energy releases it back gradually. Two things follow that examiners love. First, the oxidation state is not decoration — Eu²⁺ gives a strong afterglow while Eu³⁺ gives almost none. Second, the delay is what distinguishes the vocabulary. Fluorescence ceases within nanoseconds of the exciting light being switched off, as with a highlighter pen under a UV lamp; phosphorescence, more precisely persistent luminescence, runs on for seconds to hours because the energy sits in traps. Keep a third mechanism separate from both: retroreflective road studs and cats' eyes look bright at night but emit nothing of their own — they simply bounce headlight beams straight back at the driver.

You are not expected to have met SrAl₂O₄ before. The question is fully solvable by elimination on two chemical rules. Rule one: a phosphor needs a colourless, wide-band-gap host — typically an oxide, aluminate, silicate or sulphide of a main-group or alkaline-earth metal. Options (c) and (d) build on copper(II) compounds, and copper carbonate is green and copper sulphate blue for exactly one reason: the Cu²⁺ ion absorbs visible light. An ion that absorbs visible light cannot be the thing that emits it — it quenches. Two options fall in a single stroke. Rule two is the discriminating fact: the activators that produce visible luminescence in modern materials are overwhelmingly rare-earth ions. Europium, terbium, dysprosium, cerium and yttrium compounds sit inside fluorescent tubes, white LED coatings, television and monitor pixels and X-ray intensifying screens. Of the four options, only (a) contains a rare earth at all. The trap is (b), and it is beautifully constructed, because the pigment that really did make glow-in-the-dark stars glow was a copper-doped zinc compound — but zinc sulphide activated by copper, not copper sulphide doping zinc oxide. A candidate who half-remembers 'something with copper and zinc' walks into it at speed. The one line worth carrying into the hall is that the modern night-glow pigment is the rare-earth one; the copper-and-zinc one is its obsolete predecessor.

• The pigment is SrAl₂O₄ doped with Eu²⁺ and Dy³⁺: europium is the emitting centre, dysprosium supplies the electron traps that store energy and release it slowly. Peak emission is about 520 nm (green) and excitation runs from 200 to 450 nm, so ordinary daylight or room lighting recharges it.
• It was developed in 1993 — the discovery credited to Yasumitsu Aoki of Nemoto & Co. and patented by that firm in 1994 as US Patent 5,424,006, 'Phosphorescent phosphor' — expressly to replace luminous paints that depended on radioactive promethium.
• Against the older copper-activated zinc sulphide (Cu:ZnS) it is roughly ten times brighter, glows about ten times longer and costs about ten times more; it is now the brightest and longest-lasting phosphorescent material sold commercially, marketed as Super-LumiNova and as Seiko's Lumibrite.
• SrAl₂O₄ itself is a pale-yellow monoclinic powder of molar mass 205.58 g/mol and density 3.559 g/cm³, insoluble in water and very slightly basic at about pH 8. It is non-toxic and chemically inert, which is what makes it acceptable on toys, exit signage and watch dials.
• Oxidation state decides the effect: Eu²⁺ gives a strong afterglow, Eu³⁺ almost none. The same material is also mechanoluminescent — Eu-doped strontium aluminate emits light when mechanically stressed, and is being trialled as a crack-and-stress indicator in structures.

• Choosing (b) because you remember copper and zinc in the old glow pigment — it was copper-activated zinc sulphide, Cu:ZnS, not copper sulphide doped into zinc oxide
• Confusing phosphorescence with fluorescence: a fluorescent material goes dark the moment the exciting light is removed, a phosphorescent one does not
• Assuming that anything glowing in the dark must be radioactive — strontium aluminate stores nothing but ordinary light and is chemically inert and non-toxic

BPSC asks this as flat compound recognition — one line, four formulas, name the material — and this paper leans hard on applied everyday materials chemistry of exactly that kind: the glow pigment here, the transparent conductor of smart film at Q133, the anti-knock petrol additive at Q137. UPSC almost never asks you to name a compound outright. It asks what a material does or why it is scarce: the role of the phosphor coating in a fluorescent lamp in 2010, the CFL-versus-LED comparison in 2011, and rare earths recast as a supply-chain question in 2012.

• practice — not a real PYQ

  The phenomenon in which a substance continues to emit light for a considerable time after the exciting radiation has been removed is called

  • (a)Fluorescence
  • (b)Phosphorescence
  • (c)Incandescence
  • (d)Chemiluminescence

  Answer(b) Phosphorescence — the emission persists because the absorbed energy is held in traps or in a long-lived excited state. Fluorescence stops within nanoseconds of the source being removed, incandescence is light from a hot body, and chemiluminescence is light from a chemical reaction, as in a glow stick.

• practice — not a real PYQ

  Europium, terbium and dysprosium, widely used as activator ions in luminescent materials, belong to which of the following groups of elements ?

  • (a)Actinides
  • (b)Lanthanides, commonly called rare earth elements
  • (c)3d transition metals
  • (d)Alkaline earth metals

  Answer(b) Lanthanides, commonly called rare earth elements — the fifteen elements from lanthanum to lutetium, usually grouped with scandium and yttrium. Actinides are the radioactive series from actinium onward, and strontium, the host metal in this pigment, is the alkaline earth.